According the Haynes Manual, they say to use the same gear lubricant for convential and limited slip differentials.

Should I be putting an additive in with my 85W-90 gear oil on my Limited Slip H9 3.55 rear-end ('89 F-150)?

Thanks,
Kyle Kruszewski

 

There should be a tag on the diff that specifies fluid. And you should definiatly add the Motorcraft friction modifier. I believe it's under $5 a bottle.
I would suggest it even with high dollar synthetics that claim it unecessary.

 

I got the same rear gears that you do and I used the 75W-140 Mobil 1 synthetic. It was pretty expensive, though, like $15 a pint. They said the friction modifier wasn't needed but I still went ahead and added some.

 

Do you think that the Motorcraft Friction Modifier will cause any problems with other brands of fluid?

What is the recommended off brand?

Any ideas where to get them, none of my local stores carry the 90w-140..

Can you belive 20.-- a quart from Ford? WOW