Lucas 75W140 Synthetic w/MC friction modifier in 9.75 rear diff. Was starting to get a little gear whine or drone at 40K. Changed to lucas and now it's quiet.

 

Amsoil or Red Line.....both good products....

 

do you need friction modifier in the front?

 

Not unless you have a front limited slip. I am not aware of any factory installed, so it would have had to have been done aftermarket.

 

I use Amsoil in both front and rear in my truck..

 

Quaker State 75-140, good stuff so far. I had RP in before, drained because the cover was leaking pretty bad. Quaker State was readily available so went with it. PS, use a Fel-pro cover gasket, I used silicone and it leaks pretty easy. At least the paper gasket, you can take things apart, drain, clean the surfaces, apply gasket, bolt the cover back on and refill in shorter time. I don't have all day to let the silicone cure before bolting back up and refilling.

 

Advise a synthetic because it really will reduce rolling resistance and flows much better cold. FMs as, if and when you need it. Some syns say they don't need it but do and some don't say they do but don't. Whether they do or don't seems to be a combination of circumstances, such as the type of LSD and the oil.

I just add the oil drive it around a little (figure 8s in the parking lot) and you'll know. Add the FM an ounce at a time, try again and repeat until the chatter stops. FMs reduce the oil's oxidation resistance and decreases viscosity a little so I try to add only as much as is needed. The negative effects of FMs aren't always an issue for people and less likely to be one if you don't tow a lot or tow heavy.

Interestingly, my Auburn is starting to chatter a little after about 20K miles. Auburn requires non synthetic oil with FM. My intention is to drain the oil and have it tested. I am tempted to try some synthetic in it short term, despite what Auburn says. I had a great conversation with their engineering staff about it and they explained why they recommend mineral oil, and it makes perfect sense, but I am interested to see in the syn alone will leave it with a chatter or not.

 

I'm using the RP 75-140 without modifier and I'm really happy with it, no chatter, no drone. I didn't add any modifier since the RP said it didn't require it.