Fact or fiction on Changing original gear

[Anthony C.]

Hi all I just purchased a 71 4 speed numbers matching 2 owner car with 33k original miles.. The car is not perfect needs paint, interior and minor body work. The motor and transmission runs great, however I was going to change the gear oil in trans and diff being it was never changed before.. while talking to some local car guys they told me to leave it alone I could cause more damage than good… would love to hear your opinion on this .. thanks

[Bill B.]

Re: Fact or fiction on Changing original gear

Anthony,

I am not an expert, but my research on the best grade of oil for the Muncie 4-speed indicated NOT to use the GL-5 spec, as that could have long term detrimental effects on the bronze synchro rings. I used a GL-4 (still available from Sta-Lube) in my unit. As far as the differential, the recommendation from a member who rebuild these indicates Eaton does NOT recommend a synthetic base oil for posi units (to which your '71 came as standard). Gary R. recommends a Lucas 85-140 gear oil, with an added two bottles of the GM Posi friction modifier.

[Mark E.]

Re: Fact or fiction on Changing original gear

I've heard about the risk of clutch slippage after flushing and changing ATF in a high mileage automatic that haven't been serviced before. I've never heard about (nor experienced) any issues with changing gear oil in a manual.

[Kenneth B.]

Re: Fact or fiction on Changing original gear

Dont see why it would hurt. Believe GM recommends changing it after so many years/miles at least on cars & trucks.

[Gary R.]

Re: Fact or fiction on Changing original gear

If you have differential issues it is not from changing the gear oil, unless the wrong oil is installed. So yes, change the oil. Do not use synthetic oil regardless of all the special oil claims, you just need a good 80/90wt gear oil. I use Lucas 85-140 in cars not used below 35*F. Actually, I have driven my car in the winter between 20-20* f without any issue. You can use GM or other GL5 oils. I use the GM additive and use 2 bottles as it's not the same as it once was.

I almost always add a drain plug when I rebuild a diff and it makes it a lot easier to change out the oil. I change out the Lucas every 8-10k miles.

While you're changing the oil check the endplay in your inner axles.

[Duke W.]

Re: Fact or fiction on Changing original gear

The original Mil-L-2105D spec from the sixties was replaced with the commercial API SAE 80W-90 GL-5 spec sometime in the seventies. And these apply to BOTH the manual trans and axle. Positraction axles will also require at least one 4 oz. bottle of friction modifier that is available from GM, Ford, Mercedes, BMW and other OEMs that offer clutch type LSDs. You can also buy from NAPA, NCB 4285 V.

What does your 71 owners manual and AMA specs (download from www.gmheritage.com )?

The axle does not have a drain plug, and I don't think your trans does either, so you need to get a good suction pump, and you should change immediately after a drive that fully warms up both trans and axle.

GL-5 is absolutely REQUIRED for the highly loaded gears in hypoid axles. GL-4 did not exist back when your car was built, so GM used the same oil in manual transmissions as in hypoid axles, Mil-L-2105D and later the new API spec, SAE 80W-90 GL-5.

GL-4 MAY be okay in a vintage manual transmission. I don't know because I've never seen a long term durability test from a reputable source and likely such a study doesn't exist and probably never will. Think of the old expression" "if it works don't fix it'.

The "yellow metal corrosion" with GL-5 is a myth promulgated by the boutique oil industry. It is possible, but only at temperatures WELL ABOVE the typical operating temperatures of manual transmissions. I've talked to guys who have overhauled countless manual transmission and not one ever said the found any corrosion damage. Mechnanical wear, you bet... corrosion, NO!

One other note.. . as is the case with engine oil buy the API spec, not some specific "brand". Brand makes no difference as long as the package has the API "donut" with the spec you are looking for. These are all generic products.

Duke

[Anthony C.]

Re: Fact or fiction on Changing original gear

Thank you duke that was helpful

[Anthony C.]

[Anthony C.]

Re: Fact or fiction on Changing original gear

I've heard about the risk of clutch slippage after flushing and changing ATF in a high mileage automatic that haven't been serviced before. I've never heard about (nor experienced) any issues with changing gear oil in a manual.

thank you mark

[Anthony C.]

Re: Fact or fiction on Changing original gear

If you have differential issues it is not from changing the gear oil, unless the wrong oil is installed. So yes, change the oil. Do not use synthetic oil regardless of all the special oil claims, you just need a good 80/90wt gear oil. I use Lucas 85-140 in cars not used below 35*F. Actually, I have driven my car in the winter between 20-20* f without any issue. You can use GM or other GL5 oils. I use the GM additive and use 2 bottles as it's not the same as it once was.<br />
<br />
I almost always add a drain plug when I rebuild a diff and it makes it a lot easier to change out the oil. I change out the Lucas every 8-10k miles.<br />
<br />
While you're changing the oil check the endplay in your inner axles.

thanks Gary I do not have any issues with diff or trans but was concerned about putting new gear oil in after 50+ years and creating slippage