I did a search and didn't find an answer :(

Anyways, I'm looking at a good posibility of "having" to run a NP203 tcase complete for a while:(

I don't have the FSM and in the past I've heard everything from motor oil, gear oil, and ATF. From what I recall, gear oil was NOT right due to the chain portion of the NP203.

So, what fluid IS used to fill an NP203: motor oil or ATF?

Hopefully it will only be a temporary thing while I get the NP203 to NP205 adapter made up... and nope, it won't be a ORD clone entirely:p (with luck, I'll be using just their plates;))

Thanks in advance,
John

Plain old motor oil.

Originally posted by big85blazer
Plain old motor oil.

Any particular weight?

10w-40 is what i have in mine

I believe the factory manuel states the use of 10W/30 or 10W/40.
I have used a 203 for more than 225,000 miles gennerally with Redline 10W/40 with some moly Kalgard additive with no oil related failures.

Jason :)

30 weight with detergent is what the manuals say on the older trucks. I run 10w40 in both of mine same stuff I run in the engine.
Really not a bad case other than it weighs about 250lb. It has the biggest chain ever to come on a 1/2 ton. Part timming them is the worst thing you can do to them. The chain slinging oil is part of the lubrication process. There is a reservior in the tail shaft housing that collect slung oil. that oil is picked up by a Pump (small gear simular to a speedo gear and is located right next to the speedo gear) to get oil up to the tail shaft bearing. The tail shaft has a hole in it that the oil will travel up and lubricate both the set of bearings where the tail shaft and range box output shaft join and the top chain gear roller bearings.
Any whay that's why converting them causes problems. If you convert overfill the case some and every fill up slap the case into LOC (hubs unlocked) for a few miles to get some more oil into the reservior. Over filling will let the oil slosh into the Reservior some.
I'm getting ready yo do a review on the Mile Marker shaft replacement part time kit. It will be at www.coloradok5.com when I get it done. I will also cover their cheaper kit as well. I run double lockers so I had to convert mine.

I've been running 90W in my 203 box on my doubler, should i change up to 10-30 even though there is no chain?? Whats better for the gears?

Since you are only using the 203 reduction box stay with the 90W in it. You are only lubing gears.