M20 Tranny oil change interval

[David K.]

Gents;
My 65 L76 was restored in 1983 at 60k miles and currently has 76k miles. Car shifts just fine, however; I have no idea when, if ever, the oil was changed in the transmission. Sould I leave things as is, or would it be prudent to change. What type of oil should be used? ......... can it be done easily on my own (procedure?).
Thanks in advance !

[Duke W.]

Re: M20 Tranny oil change interval

GM didn't have a recommended change interval for manual transmission or axle oil back in the day.

I recommend at least every 30K miles, and time is not an issue.

The oil spec was Mil-L-2105D that was replaced by SAE 80W-90 GL-5 gear oil in subsequent years and brand makes no difference. Just make sure the bottle has the API "donut" with that information. Positraction axles need at least one 4 oz. bottle of LSD additive (NAPA NCB 4285V). The transmission capacity is about 2.5 pints and the axle about 3.7 pints.

Axle oil changes are particularly important for Positraction axles. Clutch wear particles contaminate the oil and eventually form a sludge in the bottom of the carrier housing.

If your transmission has a drain plug the change job is easy, but at some point it was eliminated from Muncies. If no drain plug you have to siphon or pump the old oil out.

Given that you only have about 16K miles on the oil it probably doesn't heed changing, but if you are itching to work on the car, why not? Do both the transmission and axle.

Duke

[Mark E.]

Re: M20 Tranny oil change interval

GM didn't have a recommended change interval for manual transmission or axle oil back in the day.

I recommend at least every 30K miles, and time is not an issue.

Duke

Why isn't time a factor? Is gear oil susceptible to water absorption over time?

[Duke W.]

Re: M20 Tranny oil change interval

No, water is essentially insoluble in oil, and even if some condensate develops during storage, it will boil off when the car is next operated, and gearboxes don't suffer the contamination problem of engines, so there is really no need to have a maximum time interval.

But a max time interval should be used for engine oil (above reason) and brake fluid and coolant. The clock runs whether the car is in use or not because brake fluid slowly absorbs moisture over time, and the coolant corrosion inhibitors are consumed over time whether the engine is running or not.

Duke

[Joe L.]

Re: M20 Tranny oil change interval

GM didn't have a recommended change interval for manual transmission or axle oil back in the day.

I recommend at least every 30K miles, and time is not an issue.

The oil spec was Mil-L-2105D that was replaced by SAE 80W-90 GL-5 gear oil in subsequent years and brand makes no difference. Just make sure the bottle has the API "donut" with that information. Positraction axles need at least one 4 oz. bottle of LSD additive (NAPA NCB 4285V). The transmission capacity is about 2.5 pints and the axle about 3.7 pints.

Axle oil changes are particularly important for Positraction axles. Clutch wear particles contaminate the oil and eventually form a sludge in the bottom of the carrier housing.

If your transmission has a drain plug the change job is easy, but at some point it was eliminated from Muncies. If no drain plug you have to siphon or pump the old oil out.

Given that you only have about 16K miles on the oil it probably doesn't heed changing, but if you are itching to work on the car, why not? Do both the transmission and axle.

Duke

Duke-------


During the 1963-69 period, the only Muncie 4 speeds that originally had drain plugs were the M-22 variants. From 1970-74 all Muncie 4 speeds had drain plugs.

1963-82 Corvette differentials never had drain plugs.

[David K.]

[Duke W.]

Re: M20 Tranny oil change interval

It's best to change transmission and axle oil, hot, which will stir up particles from the bottom before they settle out, again.

Duke