I had a Eaton posi trac installed on my 2009 Ranger 3.55 gears 28 spline 8.8 diff. 80w-90. Eaton recommends mineral oil so I tried 80w-90 Motorcraft and added friction modifier. It lasted 5k before chatter started. Then tried Valvoline 80w-90 with and added friction mofidier and it lasted 5k and started to chatter. So called Eaton techs, one said to try 75w-140 syn and don't add any FM . Another said 75w-140 would be not good for Pa winters try 80w-90 syn with FM . Both tech recommend Mobil1. So question is , can I use 75w-140 in Pa in the winter ? Also, can you mix say 80w-90 and 75w-140 Mobil1 ? and what exactly is chatter and how bad is it for the diff ? And if anybody uses a Eaton posi please give a recommendation for rear diff gear oil. Thanks.