SodaLuvr

What gear lube should I use? Dad's van needs a change. Manual says 80W90.

Should I use?

75W90
75W110
75W140

I don't plan on touching it again, ever, so this fill will be its last, or dare I say, lifetime fill.

Amsoil?

Mobil 1?

Redline?

Is thicker better for gears?

iTimmy

Use what ever the vehicle calls for, a lot of people never change gear oil and never have a problem. The only advice I will offer is that if the vehicle has a limited slip differential make sure you put limited slip additive in with the gear oil. Due to the extremely low rate of failure with gear oils I always just used what my oil supplier suggested, I don't really think a premium gear oil will make a bit of difference over a regular gear oil.

bigman

people usually change gear oil if they beat on the car, if it's making noise, or if there is a pinion or cover seal leak. Other than that, i've seen 30 yr old cars with the factory oil in them.

Some advice-
What type of car is it. If it's a GM with LSD, you have to put a bottle of GM(GM ONLY) Limited Slip Additive. Make sure when you get that cover off they you clean the differential with brake cleaner. Also go at that cover with break cleaner and a wire brush. Get all the metal shavings off the magnet and scrape the gasket off completely. You dont want to have gasket sealing problems. Once thats all done, put a bead of rtv all the way around and around the bolt holes as well. That should be it. Oh and remember to cross tighten the differential, like you would wheels on a car.