Old habits die hard Lloyd. Kinda like the 3000 mile oil change. Before we had injected engines we had carbs. Carbs were sloppy at best. There was a lot of fuel that ended up in the oil especially if towing. You used a heavier oil because it gave you an edge on the viscosity dropping from the thinning effects of the gas in the oil. That's why a lot of folks used it. Some want to claim that you needed a heavier oil in high heat when the exact opposite is true. High school general science class would have taught you that a thinner fluid absorbs heat faster and cools quicker than a thicker fluid. That pretty much describes what oil does in the system. It gets pumped into the engine where it absorbs heat, goes back to the pan where it's released.
Gotta love the recommend of a 10w-30 over a 5w-30. At full operating temp do you know what the difference is? Absolutely nothing, they both are SAE 30 viscosity at temp. The benefit of the 5w base oil is that those oils are made with a higher Viscosity Index base. The 10W, not being a volume seller, is made from low VI base oils and polymers are used to make the spread. Polymers, while better than those of the 90's, are still a sludge maker. Using a 10w-40 is asking for problems as GM found out and ended up having to replace several thousand engines. The recommendation makes zero sense.
FWIW, you'll be seeing oils even thinner than the Xw-20 as you'll see 0w-10 ans 0w-0 oils in about 5 years. Instead of viscosity keeping the parts from rubbing on each other, we now use film strengths and additives that leave dry film lubes on the parts. Another FWIW, NASCAR teams don't use bottled oil like you and I buy. They make their own. Depending on the race track determines the additive package and viscosity At the heaviest, an SAE 20 and a lot of the races use an SAE 0 oil. Yep, it's about like water yet the engines live at RPMs and heat that would kill your stock engine fairly quick.
Another FWIW. All of the auto makers in the USA are memebrs of ILSAC. ILSAC is an organization that determines engine technologies and especially the oils we use. API is the enforcement arm of ILSAC. Any engine designed for the USA market starts with the lube system. That's the absolute first thing that goes into the specs. Since the lube is the first and most critical thing to an auto maker, wouldn't it make sense that the auto makers recommendation is dead on as that's what the engine is designed around? It's hard to out guess an auto engineer that's been doing it most of his life.
