Can I use this?

[Ronald F.]

I need to top off my '67 4.11 posi Differential lube. I don't know what's in it now. I have a left-over quart of Red Line MT-90 synthetic lube. Can I use this? If not, what would be the best? Not knowing what's in it now, would I need to drain what's in it first? Thanks!! Ron

[Gary R.]

Re: Can I use this?

Ron, not knowing what you may have in the diff or for how long I would replace it. I don't use synthetic oil in any diff I build or in my cars. This can be a life long debate among some and that is not where I'm heading with this. What I would do if it were mine, suck out the old oil( I tap drains in my builds) use Lucas 85-140 or 90wt if you like and (1) bottle of GM limited slip additive. Top it off with the diff level and the oil just weeps out the hole. Then go for a nice drive, go to an empty parking lot and do the ole figure 8's at low speed to work the additive into the clutches. If your diff is original then it has the solid steel clutches, which are the best, and you should be all set. If you have any problem let me know.

[Ronald F.]

Re: Can I use this?

Thanks, Gary. I looked up the Lucas oil and it says it "Includes friction modifier in formulation to conform to the needs of limited slip differentials." Do I still add the GM additive?

[Gary R.]

Re: Can I use this?

Yes use the GM additive. Depending on how tight the posi is setup you may need a second bottle but start with one for now.