Yeah, I wonder how much that shop guy really knows or understands about gear oil. Gear oil viscosity and motor oil viscosity are not comparable. 75W-90 gear oil is about the same as 10W-40 motor oil.

Here's a basic primer on the difference: http://www.widman.biz/uploads/Transaxle_oil.pdf

Quote
Viscosity:
First it should be noted that the SAE motor oil viscosity chart (J300) is different from the SAE gear oil
chart (J306). A SAE 40 motor oil has approximately the same viscosity as one of the thinner SAE 90
gear oils, while the SAE 50 Motor oil is similar in viscosity to the thicker SAE 90 gear oils.

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