Piston Slap: One or Three Shells for your classic Fiat?

Piston Slap shell oil

Leo writes:

I have a 1947 Fiat Topolino and it calls for Triple Shell oil above 50 degrees and Single Shell oil below 50 degrees. What are triple and single Shell oil? And what oil should I use?

Sajeev answers:

I’ve spent far too much time trying to learn more about vintage oil technology in order to answer your question. Why? Because it’s all totally, wholly irrelvant to your oil change needs in the year 2022. Don’t get me wrong, I admire the fact that you actually read your owner’s manual, but the writers of said publication never considered an evolution to modern oils. Perhaps this evolutionary infographic from the Petroleum Quality Institute of America can show why what you read is now irrelevant:

The Petroleum Quality Institute of America

So let’s instead focus on what’s available for sale today. I’ve already sung the praises of Shell Rotella T4 for cars made before the introduction of catalytic convertors, as this diesel engine oil is ideal for such applications. But diesel oils in general are fairly affordable and readily available, be it from Shell, Mobil, or Chevron. Heck, even Walmart has its own brand of Super Tech diesel oil, and the quality is likely nothing to sneeze at.

But since you mentioned Shell, let’s stay there for a moment. Shell has a full synthetic (Rotella T6) and a “proprietary, synthetic blend formula” in its T4 diesel oil. Your owner’s manual suggests oil is seasonal, but I doubt that has any merit with today’s formulations. While running Rotella T6 will likely aid in cold start lubrication, a full synthetic might be more likely to cause oil leaks on an older engine with marginal gaskets. Odds are the semi-synthetic T4 is more than adequate for your needs, is less likely to exacerbate a leak, and will be a few bucks cheaper to purchase.

More to the point, I wager that Shell Rotella T4 (or its equivalent from Mobil, Chevron, or even Super Tech) is far better than the combined benefits of Fiat’s recommendation of Three Shell in the summer, with One Shell in the winter. Of course I need the pertinent technical data, but that’s seemingly impossible to find for the older stuff. No matter, I’d be blown away if Rotella T4 didn’t do justice that to its predecessors.

Bottom Line: Ignore the 75-year-old owner’s manual, just run a modern semi-synthetic diesel oil in the closest weight to the original stuff (presumably 15W-40).

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