67 rear end grease

[Jeff B.]

Wanting to change the lubricant in the rear end of my 67 w/positraction. What is today's preferred choice, synthetic or non-synthetic?

[Gary R.]

Re: 67 rear end grease

Well first you don't use grease.

Use non synthetic gear oil and posi additive. There are many slick brands sold today with additives in them. I can tell you what I use and have for decades, Lucas 85-140 gear oil for use in temps 30*F or higher, or 90 wt. Some claim the 85-140 is too thick, never been a problem in the literally 100's of diff's I built.

I use the GM additive, it is not the same as it once was and I'm not speaking of the whale oil days. The current stuff I use 2 bottles- although I tune the posi's and 1 may still work, but 2 works.

What sold me on the Lucas was I opened up a diff I built 10 years earlier. It was never installed in a car, just turned by hand once in a while then just let sit for years. I expected to find the oil had run off the bare iron parts and thought I would see some flashing. However, the oil was still clinging to the exposed metal. There was no flash, no issue with the oil, it appeared as clear as the day it was installed. I haven't seen that with common gear oil.

you can use any quality oil but stay away from synthetic, it was never recommended by Eaton. If you have steel clutches, it probably will work but if you have the new fiber coated junk that is sold today it won't
.

[Jeff B.]

Re: 67 rear end grease

Gary:
Thanks for your response. I highly respect your opinion. Sorry about saying "grease". That is very old school terminology we used to call lubricants used in rear ends. The rear end lubricant has not been changed in the 45 years I've owned car, and could still be original. Am not having any problems or issues out of it, but just decided it could use changing. I like your suggestion of the 85-140. Thanks again for your advice. Much appreciated.

[Duke W.]

Re: 67 rear end grease

The original Mil-L-2105 spec was replaced with the API spec, SAE 80W-90 GL-5 back in the seventies. Brand makes NO DIFFERENCE! Buy the API spec, and you can buy the Positraction additive from NAPA or other parts stores.

85W-140 is overkill. It's designed for large truck axles that turn lower revs. If GM thought it was the best choice for passenger cars, they would have so stated. In a passenger car axle that turns higher revs it will increase windage drag.

Duke

[Gary R.]

Re: 67 rear end grease

Jeff
No problem, understood. Sometimes, not a lot, I do find that grease has been added in so you never know.

I have also opened up diff's with what was once oil and baked on everything, you would swear it was powder coated. Hopefully yours isn't like those.

Good luck with the oil change, it is a pain to get it out at times. It's the reason I usually tap a drain plug in non ncrs diff's. Makes life way easier changing out the oil.