NP203s and 205s are anything but interchangeble(swap t cases and be good to go).
I have a cheap part time kit that I'm just waiting to go out, and I would like to swap to a 205 when it does, but it would actually be cheaper and easier to install the good part time kit that get a good used 205, driveshafts, adapter, C6 tailshaft, crossmember, ect...
I know all this, because I was trying to do the swap, and it was a no go.

 

If you had a truck that originally had an NP203 that was missing or broken it would be cheaper and less hassle to replace it with another one. Swapping in an NP205 would be an upgrade if you want to fabricate a new mount and then lengthen or shorten the driveshafts. Time and $$$$. I'm talking about a divorced transfer case swap. I don't believe they are interchangeable without some custom work. Anyway, you should be locking in your hubs every 200 miles in any 4WD truck, be it full or part time to lubricate the front diff and hubs.

 

Some oil company sell SAE 50w gear oil which contain the special gear oil additives.

Do not use multi-grade engine oil (such as 10w40, 20w50, etc...) due to the polymers in the engine oil, which will cause foaming, and which will create deposits in the transfer case.

Use straight SAE 30w or SAE 50W on your tranfer case (NP 203 and NP 205).

 

guys, I was talking about a married set up and you don't have to get an adapter the 203 and 205 use the same adapter and input shaft spline count. If your talking divorced I don't think it would be very hard to find a cross member from a donor. The only things that might have to be change is the drive shaft lengths and the shift linkage and if you find a donor truck you can get all that off the donor. Also where does it say you have to put your truck into 4X4 mode every 200 miles. I've never seen this in any shop manual and most people that don't live in a climate that dictates you do, don't. If you are building a trail rig that will see limited road use or you live in Alaska you might do this but for most of us the trucks will only see limited 4X4 use. If I were going to use a chain drive case it would be a newer one like a 1356 or a 4407 or maybe a new venture out of a super duty. They use the same input shaft and adapter ( with some mods) they are just over 100lbs. they are lower geared, they use a magnesium case, and they are part time to start with. The also have a aircraft quality chain to start with.
The 203 was a OK case at the time but their is alot better options now.
Leadmic