Hi I have a 1997 dodge B3500 van, converted into a campervan, It is time to change the differential oil, and I was thinking about putting some synthetic oil in this time, with the hope of better protection, and maybe longer life of the parts in there, has anybody done this before, I looked at the different brands, there is, Amsoil, Purple , Lucas, Mobil 1 etc, prices are fairly similar, some more than the others, I also checked everywhere in my manuals, and not sure how many quarts I will need to buy, Does anyone have the proper amount needed to fill the differential, thank you for all your help,, ,I have learned a lot from this forum,,,,,,

 

This is an expert opinion (by no means) but were it me I'd stick with whatever the factory recommendation is for your rear axle, this is a low maintenance item but something you don't want to neglect for sure... I learned the hard way about 5-6 years ago! Fresh lube is a good thing regardless of whether SAE or symthetic, again, maybe just stay with what's recommended imho

Mine is a 99 3500 van too and has the Spicer/Dana 60 rear axle. You can find your axle listed on the underneath side of the hood sticker, just over the battery or near there.

My owners manual doesn't provide the amount of fluid required but does recommend API GL-5 compatible SAE 90 for the Dana 60 - covers all temperatures. Fill level should be at 3/4 inch below the fill plug, I'd think 3 or 4 pints ought to be safe to get it filled but hopefully someone else with better knowledge will add their opinion, also the considering the 4-6 onces of modifier. Think it will also recommend gear oil additive/ friction modifier for limited slip fyi.

 

I have had 3 mopars and they have all used 75w140. I could call the dealer to see what they currently recommend as it could have changed since the owners manual was published. I always buy extra and either keep it or return it depending on how much is left over. Sometimes it can be challenging to squeeze out the whole bottle depending on what is in the way. My 1991 B250 has a trunk so you can only get about 3/4 of a bottle out and then have to combine all the part bottles to finish the job. Generators can get in the way too...
Steve

 

For the trucks, standard recommendation is 80W90 for regular service, and 75W140 (synthetic) for heavy duty use. (towing/hauling.)