Just curious because I'm looking at having my rear pinion seal replaced and the place I went to the guy said he feels it was because I was using 75W90.
Vent is fine, level is between add and fill on my Mag Hytecs...getting kinda tired of having a leaky seal back there. He told me he thinks that heat from hauling/driving caused the seal to go, and that 75W140 with a new seal will fix it...
So what runs hotter? 75W90 or 75W140?

 

It doesn't matter which fluid you use, because I have been running synthetic 75-140 since 15K miles and changed it every 15K and the front leaked at 37K, the rear started leaking at about 75K. It is crappy seals from AAM that are the problem.

 

Plus 75w140 is REALLY thick to be running in Edmonton in the winter. We have similar weather and synthetic 75w90 is fine unless you're towing SUPER heavy and its 38 degrees (or 100F) and you're booting it 130 km/h loaded.

Just my over rated 2 cents.

 

I would doubt it would be temperature related. The warmest I've seen my rear diff. with Amsoil 75W90 in it while towing approx. 9000 lbs in 30*C was 190*F on my gauge with the same diff. cover you have.

 

I would think that lack of lubrication from heavy fluid in cold temps is just as likely to damage seals (not to mention other stuff) as high temps from thinner fluid. I would stay with the lighter stuff in colder climates. Strictly towing etc. in warmer climates makes sense to use the 75w-140 but I don't think its practical for most guys. I read a lot about leaking seals so I'm also going to assume its from a crappy quality seal..