Piston Slap: Guidance on Flat Tappet Oil


Jack writes:

How about some guidance on motor oil for us old farts who are still driving flat tappet engines like my ’74 Corvette L82?

Sajeev answers:

Woo-hoo!  This is a pretty easy answer to give, as 1975 was the first year for Corvettes with a catalytic converter. All pre-1975 vehicles will benefit from the added zinc content available in oils designed for diesel engines. Yep, it’s pretty much that easy!

You can buy modern oils and a standalone zinc additive, and there are modern “racing oils” with a loyal following. But the standby stuff for diesel engines is both commonplace and downright affordable, which makes it rather hard to beat. Oh, and they come in synthetic (if you are confident in your gasket’s sealing) or conventional oil (if you are not).

Things are tougher if you have a 1975+ vehicle with flat tappets and a factory-fit catalytic converter. You might not want a high zinc content, as ash buildup can destroy the catalyst. This happens most likely due to engines burning oil, and I assume many a Malaise Era automobile had their cats defeated with a rod/broom for this reason alone. Engines that don’t burn a drop of oil are unlikely to experience this problem in their catalyst, and might be able to get away with modern gasoline engine oils.

“Pancake” Catalytic Converter, typical of vehicles from 1975 to 1980(ish). General Motors

So let’s formally ask ourselves this question: What if Jack had a 1975 Corvette with its factory cat? Or one of the many other flat tappet/catalytic converter vehicles still on the road? First off, that’s impressive, and I thank you for saving a piece of Malaise Era history.

Running a modern synthetic oil for gas engines with high mileage additives ensures maximum protection for flat tappets and minimal leaks (as synthetic oils can exacerbate a leaky gasket). You might not have a leak yet, but unless your motor has low miles and gaskets that still seal like new, always opt for the high mileage synthetic oil. This might not be as good as diesel oil for the flat tappets, but it will likely do an adequate job reducing engine friction while ensuring a healthy catalytic converter.

Then again, do you even need synthetic oil? I reckon most of these 1975–85 vehicles are driven infrequently. Maybe just ordinary, not-synthetic oil is good enough. Will you drive it enough to wear out an engine with flat tappets? Your call, but just know you have options.

Have a question you’d like answered on Piston Slap? Send your queries to pistonslap@hagerty.comgive us as much detail as possible so we can help! Keep in mind this is a weekly column, so if you need an expedited answer, please tell me in your email.




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    It is simple 15w40 Mobil on Racing oil.

    It is available at Walmart and has the levels of zinc and phosphorus that the oils originally had.

    The damage to a converter is minimal and with the limited driving you are not going to kill it anytime soon. Most need replaced by this point anyways.

    The priority is to protect the lifter to cam and not wipe it out. Converters are much easier to replace.

    I also use it for a mid engine car where oil temps can climb. This is a benefit of the synthetic.

    I was taught valuable lesson by a fellow vintage racer who is a chemist at Mobil oil one day ….
    There is a little insignia at the bottom of the oil can signifying that it is either ASE approved or “Official of NASCAR” . Synthetic or not the latter indicates that it contains Phosphate and sink additives. The phosphate opens the surfaces to allow the sink (a surface lubricant) to adhere to bearing surfaces
    Mineral oils (non synthetic) was proved years ago to be very poor in high temperature applications.
    Personally I prefer Mobil 1 0W40 European formula oil for all my classics and 80/90 high performance car.
    For ultra high performance race engines I use Mobil 1 Racing 0W50 – the wear rate on components is astonishingly low and the rate of oil degradation low ….. only change it on my vintage race engines max twice a season, rather than every race day when using ordinary synthetic oil.
    The supposed term “long life synthetic oil” has always bothered me. I never run my general usage cars for longer than 4000 miles between oil changes. Paying $30 for oil and a filter every 4000 miles makes a lot more sense than paying thousands for an expensive engine rebuild.
    I have to strongly disagree with the comment about damage to Cat converters by using oils with zink content. Think about this ….. it only applies if your engine burns oil. If that is the case time to look at that engine or at minimum head rebuild!


    Point of clarification it is 15W-50, but yes it has the correct amount of ZDDP for a flat tappet cam. Not enough to break one in, but otherwise perfect.

    I’ve been running it exclusively for over 25 years. First oil change in a 2001 truck was with it and every one since. 200k, deep gears with high RPM highway speeds, heavy off-road use. Cat works perfectly, just passed emissions and they made it run the dyno test because their OBD-2 test flagged I had changed the tune.

    In fact, I have run that oil in 3 different vehicles with zero issues, each over 150,000 miles.

    Don’t use diesel oil though, it has a different detergent pack.

    Have three cars which need high-zinc content motor oil. Been using Mobile One Racing, Penn Grade (formerly Brad Penn), Driven, or Valvoline VR1 with good results so far. Plenty of other quality brand choices, too, so probably comes down to personal preference. Like every other subject under the sun, lots of good (and plenty of bad) info available on Youtube and elsewhere online.

    Sajeev, If you tried to use a rod and broomstick on the 1975 era cats, all you woud get is a busted broom handle and a sore arm. The pellet type converters on early GMs had a plug you pulled out to drain them, but it would have been better to take them off as they were very restrictive unlike the modern honey comb style cats. Can’t believe Hyper missed that

    Frank, that’s 100% right and I should have specified that the broomstick method would only apply to later 3-way catalysts after 1981(?) that used the internal matrix design and not the pancake.

    I regularly use Mobil 10W30 diesel grade oil with great results. Its a great budget alternative as it has the higher zinc that diesels require and I can get it in a 10W30 weight. I couldn’t beat it for the price. I prefer amsoil but do use Mobil when the budget comes callin’.

    Jeff Smith, former editor of Hot Rod, this month wrote an extensive article for Hemmings Muscle Machine about oils and ZDDP. He states unequivocally that the ZDDP in diesel oil are not of benefit in non-diesel engines.

    Be aware if you have not installed a came recently you also need to use s bit of break in oil now. This additive is made to break in the cam and prevent from flattening a lobe.

    Many of my customers wipe out their cams by not reading the new instructions for today. These additives are easy to buy and are not STP.

    Many call in claiming a defective cam or soft lobe. Well the whole cam is made of the same metal and they did not break in properly.

    The use of the high content oils will keep you going as long as they are offered. Beware things often change so keep up if the oil you use is changed.

    I have said for a long time they do not have to out law old cars to get them off the road. They just take away our oils and force us on the alcohol based fuels that will eat up our fuel system. This is happening in Great Britain now. Kind of like gun control by make ammo hard to get.

    You are right on the newcam – using synthetic in new cams or new rings in a re-honed bore is a bad idea.
    Use ordinary grad 40 weight mineral oil for the 400 to 500 mile run in period, then switch to synthetic.
    The biggest problem is cam followers or lifters not rotating and freezing in place – it will instantly wreck the follower and am lobe- they have a 6 degree angle on the follower to promote the rotation.
    Synthetic oil in a newly built engine is a death sentence !

    Most pro engine builders will recommend 30 weight break-in oil specifically for the initial cam break in and then the 500 mile test.
    Regular 40 weight oil will have extremely low ZDDP and will absolutely flatten a cam lobe. It’s not just the lifters that rotate, the rings do as well.

    I caution using the diesel route as they have been reducing zinc in even that oil. Diesel oil barely meets the minimum required. A ZDDP additive is cheap and gives you the adequate levels of zinc.

    Anyone have experience with Lucas 10w30 for classic and race cars. High zinc content for flat tappets. It is a conventional oil, not synthetic.

    Yes, I’ve used it for 10 years in my 1971 Olds with no problems. I change it once a year, putting on less than 2000 miles every summer.

    This article is typical of a “writer” trying to be an engineer/scientist. The low/no ZDDP hysteria just wont’ go away. Fact is motor oil had little to no ZDDP in it for decades and engine camshaft failure was not an issue IF the owner changed the oil per schedule. The big change happened when the Big 3 automakers extended oil change intervals to lower owner costs in between the late 50’s and early 60’s – THEN camshaft failures increased! What to do. Walking back the oil change interval would have been a fiasco, not to mention legal actions. Increasing ZDDP fixed the issue by reducing the change of scuffing as the oil wore out. HOWEVER too much ZDDP is corrosive!!! So pouring a bottle of ZDDP additive in a crankase is risky!!!! When the ZDDP hysteria started up I tracked our club members cars for five years and most used a 10W30 low ZDDP oil, BUT they changed it every 1,000 – 2,000 miles (just like the days BEFORE increased ZDDP). In five years there was NOT ONE flat tappet camshaft failure! On top of that today’s low ZDDP motor oils are superior to motor oil sold in the 1960’s. Bottom line — just change the oil every 2,000 miles and DO NOT WORRY. As far as diesel motor oil goes —- due to diesel emission regulations the HIGH ZDDP diesel oil went away years ago. Although diesel motor oil tends to offer better camshaft protection, it really isn’t needed. Same for synthetics whose major advantage is tolerating high heat, great for racing, a waste of money for a Sunday driver hobby car. Just my 2 cents. Feel free to look at the SAE papers and today’s oil blends.

    I have looked at the SAE papers.

    I also trust the Engineering Physicist at Comp Cams who’s job it to make sure their products don’t fail.

    Roller tappets have been pretty common for over 35 years and that negates the majority of high ZDDP need.
    Still though, FCA had an issue with MDS roller lifters puking the trunnion bearings due to low quality oil.

    Current standard passenger oils are 800/870 Phosphorous and Zinc.

    SG (1995) were 1300/1400.

    Mobil 1 15w-50 is 1200/1300.

    Mobil 1 10w-30 is. 760/830.

    Why risk a few grand on a rebuild when high ZDDP offers better wear protection?
    Especially at start up.

    SAE papers – you should read the glowing write up about Ford’s modular engine, especially the 2V V-10. They are engineering papers, not the Bible.

    Old Bib I have to agree with you. I have used regular Pennzoil for years in my flat tappet cammed engine and regularly change at 2000 miles or less. No problems at all.

    Here is the deal.

    If you are breaking in an engine the additives are absolutely needed. Break in is the most critical time for it. It is not a matter of it but when it fails.

    Second the other issues in play are to be considered. One a broken in engine is more durable but driven daily it can fail. The reason we do not see massive wear and failures are that most of these cars are not daily drivers.

    Another factor is this. What condition your heads are in. How are your guides. Are you running double springs or heavy springs. What RPM are you running. What quality cam are you running.

    There are a number of factors in play.

    I know this is a real thing as I have seen an increase in failures and seen an increase in cam sales. But not all engines are equal and some will just flat out fail before others.

    Another failure point is bad rocker geometry. That is enough to kill a cam alone but if you have it wrong it may fail sooner with the wrong oil. Many people get this wrong and it may be the oil or just a bad set up.

    If you are doing track time you may see a failure much sooner vs the guy just going to the drive in show on Saturday.

    The bottom line is if you plan to keep a car long term start to run oil with the better content and odds are you will never have to change a cam. If you are not ticking yet you still have time generally so it is a no brainer.

    The one thing you fail to comment on that I would like to add –
    I recently had cam re-profiled by DemaCams after my Healey suffered catastrophic cam drive failure including the follower breaking up – very ugly.
    Dema suggested I got clear certification of the replacement cam followers from the supplier.
    It’s a complex business based on the Rockwell hardness scale.
    A low Hardness can ruin a perfectly good cam re-grind..
    Funny how heavy valve springs has almost become a fad. It’s all about usable rev range – fitting valve springs to manage 8000 rpm on an engine that power peaks at 6000 rpm is a pointless idea that over stresses the valve gear.
    Needless to say a lot of it also depends on the design of the cam lobes – importantly the rise and fall rate on the lobe tip. Buy a decent cam or have it ground by a highly reputed shop, and don’t skimp on the cost of re-hardening …. it’s important.

    FYI if you are not using a Synthetic you should be. There is no better oil street or race. Now prices are cheap there is no excuse.

    Even the better racing oil is cheap at Walmart if you need it. I can change mine for under $30.

    Come time for any engine work it is so much better on over all wear even just street driving. This is why nearly all new cars use it now.


    High quality break-in oil, high quality filter, the switch to synthetic. Every time.

    I keep getting flagged for using brand names, but ZDDP content absolutely matters, even on a stock cam rebuild.

    I wasted a new stock GM cam on a rebuilt 350 smog engine running parts store 10w-30 on break in. Second time I used $10 per quart (20 years ago) break in oil. Engine is still purring to this day.

    Hyperv6 hit a couple nails on the head. High lift/duration cams with big valve springs require additional insurance and are more prone to failure. Another issue is many folks these days are rebuilding blocks and heads that are old, and may have had geometry issues when built, or from sitting around on concrete floors for years. Old engine pickings are getting thin. Based on what I hear from friends and folks online I am sure that there have been process issues with new cams and flat tappet lifters, but there are many other contributing factors besides the oil. I run Lucas Hot Rod 10W-30 in our ’71 vintage 355 that is in our Nova. It has a flat tappet Comp Cams Cam/lifters and after 3 years of summer driving it’s still fine. Fingers are crossed that it lives a long life.

    I have vehicles from 3 different era’s. 1949 Merc with a EAB flathead from a 1952 Ford. 1956 Fairlane Victoria with original 312 Y block, and 1972 GMC PU with late “60” ‘s 327. I use 10W-30 Motorcraft in all of them. Change oil and filter once a year. I use Castrol GTX synthetic blend in my 2001 F150 and full synthetic in my 2017 Fusion 4 banger. All works for me.

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