I know this has been debated before, but what is the consenses of opinion about the best synthetic gear oil for the front and back diffs. I have an 04 FX4 with the 3:55 gears. And for that matter, what is the best lube for the transfer case.

 

If you don't expect to change it for a long time, by all means, use synthetic. But if you change it frequently (i.e. because of submerged axles), use something cheaper that meets the spec. Last time I changed (almost 2 years ago) Amsoil was at least a buck cheaper than the other brand (I believe it was Mobil 1) in the store I got my oil, so I used Amsoil 75W140. Other than price, I don't think there is a whole lot difference between various 75W140's.

 

I use 1/2 full synthetic and 1/2 Lucas oil stabilizer in my diffs. Just my personal choice though.

 

Don't forget a few ounces of friction modifier in the rear diff if it's a LS.

Your transfer case calls for Mercon-rated ATF, so something like Mobil 1 ATF will work if you wanted to go with a full synthetic in there.