Since my Eaton posi had a propensity to fry pretty much any gear oil I put in there, I decided to try Amsoil Severe Duty 75W-90W. Eaton recommends petrol based oil with 2-3 tubes of friction modifier. I lost count how many different brands of oil and modifier I tried, but severe chattering would return after less than 1K miles. Amsoil ended up sending me 75W-140W instead of what I ordered, but I decided to try it anyway. The chattering is barely noticable now...even without friction modifier. We'll see how it does once I get some miles on it. If it worsens, I'll try Amsoil's friction modifier. If that doesn't fix it, I'm throwing out the posi and getting a Truetrac.