Mit,
I use 15W40 chevron diesel rated oil in the engine. Rotella is good too. Whatever you used make sure it if for a diesel engine. My transmission and differential has common reservoir on both my tractors. (smaller than yours) If you are going to use a backhoe, loader or other hydraulic stuff and your 1850 has a common reservoir then you can use hydraulic oil rated for gear drive transmissions. It is available at Tractor Supply, Wal Mart, parts stores etc.. I buy mine in 5 gallon pails and it says what it is for on the bucket. It usually gives a list of brands it will be compatible with. Look for one compatible with JD 303. (pretty common). Both my tractors puff a little smoke, not much.