I guess this is more of a question for the northern guys. I put 90w synthetic in my 203 tc a couple of years ago when I changed fluid. I just figured out it should be lighter. It hasn't leaked at all. And I haven't had any problems. I installed the part time kit in 94. Should I change it to like 20w-50? I drive it in the winter in MN. I was going to try and find synthetic. My truck is a 75 f100 4x4 with a 429. By the way today I scored and bought a 79 4x4 with a 460 and a 205 tc. Hopefully that will be this summers project.

 

i use 80-90 in my 205 yr round in michigan and no issues

 

It's a 150, got it for $350 and saved it from the scraper. Wanted it for the tc. setup. Might just use the 79 frame and put the best body and my built 429 in it.

 

You must run thinner oil in the 203. This case will not oil properly with gear oil in it. Gear oil is too thick and cant get into the oil passages properly, and the rear output bearings will burn up.
Two reasons a 203 fails:
1) incorrect fluid (too heavy)
2) failure to lock hubs, or engage 4wd every few hundred miles to add lube to the reservior that feeds vital components.

This is taken from Mile Marker:

The item that needs oil the most is the rear tail-cone. It has an oil catch which the chain throws oil into. This oil is pushed by a plastic oil pump gear into the rear output shaft bearing. This oil catch needs to be refilled occasionaly, and the only way to do this is to drive in 4wd (or lock the hubs). MileMarker recommends running the truck for aproximately 10 miles every 200 miles. This is most easily accomplished by simply driving to work one day a week with the hubs locked.

The NP203 uses 10w-40 motor oil. People severly hurt their 203s by using either gear oil (90w) or transmission fluid (18w) in their cases. The gear oil is way too thick for the 203 and will not lubricate the links in the chain. Transmission fluid is too thin, and will not properly lubricate the ball bearings used in the input and output shafts. If you have used these fluids in your case, you need to drain it immediately, and fill it with the correct fluid.

Since this case is converted, the multi-viscosity oil will be fine, but I believe straight
50W was used for full time applications.
Straight non-detergent oil will foam less in this case. (no pun intended)

 

Thanks for the info. I was also hoping it would allow me to move better when its -20F outside. I will change out the oil and see how it works.