The simple explanation is that the Gear Oil ratings and the Engine Oil
ratings are on different bases. Dave is correct about some of the
temperatures used. The numbers in the ratings are not proportional to the
viscosity, i.e. SAE 90 gear oil does have about the same viscosity at 100
deg C as SAE 30 engine oil. The additive packages are designed for
completely different issues, that is the big difference. Current day engine
oils are probably not great for gear systems because they have detergents
that enhance foaming in a gearbox. In the '50s and '60s, the differences
were not as great and you could readily purchase non-detergent engine oil.
The "W" rating, by the way refers to the oil performance when cold. One
should focus on the second number to choose an oil for expected normal
operating temperature, look at the "W" number depending on your climate.
Our friend in Nova Scotia obviously needed a low one this week, while the
Florida TR drivers do not. [The guy in Nova Scotia probably doesn't need
bug spray in his glovebox though.]
Glenn C
1959 TR3A TS31684L [in total frame up restoration]
1960 TR3A TS64803L [driving, but needs work]
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