Multi weight oil in transfer case? I thought it was a no-no.

[440]

Not really a mopar question but I thought multi weight engine oil in transfer cases was a big no-no. The NP203 transfer case is chain driven unlike the NP205 meaning it doesnt need a hypoid type gear oil. Ford recommends SAE 50wt but upon contacting Royal Purple they say to run HPS 10-40wt, which isn't cheap.

I've run Royal Purple engine oil for years and finally want to switch all of the other fluids over.

[Brutalowner]

For quite some time, it has been unofficially known that the best way to keep a NP203 oiling happily, is to use 10w30, or the like. The 203 mode box has a plastic gear oil pump and an oil tunnel from the output shaft, thru the main-shaft to oil the three layers of stacked needle bearings. The range box is however gear to gear, only when in low range.
If you live in a cold climate, the thinner oil travels forward to the bearings quicker, helping decrease wear.

My .02