63 Borg T-10 gear oil recommendations

[Todd L.]

I purchased conventional 80W-90 gear oil for the transmission on a car I am resotoring, which is API service GL-5. After a little research, it appears GL-4 is what I should use. Does anyone have any knowledge of what is correct or if either one will be fine. I don't plan to drive this car thousands of miles. Any wisdom would be appreciated.

Todd

[Duke W.]

Re: 63 Borg T-10 gear oil recommendations

Look at section zero of your 1963 Corvette Shop Manual. The original oil spec for BOTH the axle and manual transmissions was Mil-L-2105B. I bet you haven't seen that around for awhile. The mil spec was cancelled and replaced by the commercial API spec, SAE 80W-90 GL-5 spec in the seventies, and that's what you should use.

Various boutique oil companies and internet "experts" claim that the sulphur-based EP additive in GL-5, which is what gives it its pungent odor "causes yellow metal corrosion", which is nonsense. Yes ,it can cause mild corrosion, but ONLY at temperatures well above the operating temperature range of an automotive manual transmission.

GL-4 didn't exist in the sixties. Beginning in the seventies some new design manual transmissions speced ATF, which is basically a SAE 70W-75 viscosity GL-4, but some designs specify a 75W-90 GL-4. Some guys think this is an "upgrade" for an old T-10 or Muncie, but I have never seen any reliable durability test data, and until I do I'm sticking with SAE 80W-90 GL-5. Make sure this is written inside the API "donut" on the label.

I've talked to guys who have rebuilt old manual transmission for years and they NEVER reported any "yellow metal corrosion". Wear, yes, corrosion, no!

Duke

[Todd L.]

Re: 63 Borg T-10 gear oil recommendations

Thanks so much for the great detail Duke!!