FF is 80w90 dino except for towing pkg. Mine has towing pkg and the manual specifies 75w140 syn was FF for that. I have always used 75w140 in the rear diff accordingly, but now I am abandoning synthetic gear oils and I need some advice. I only tow a light trailer usually (under 2000lbs). I am worried about cold-weather flow of the 85w140 dino and I am thinking of just using 80w90 dino in the rear diff, despite what the manual says. What do you think I should do? Now summer is approaching but I do not want to have to do a diff service for each season. I ran the thicker stuff in the synthetic because I was less worried about its flow characteristics and I suspect that is why the OEM recommended full-syn for the thicker gear oil in tow pkg applications. This is on a '98 Grand Cherokee with the Mopar Trac-lok limited slip & a dana44a axle.