:Thanks very much for the info, you guys are great. The smoke is white as far as I can tell. It does seem to be worse when I dont let it warm up before shutting it down, but "warm up" is hard to tell, since there is no temperature gauge, just idiot lights I'll leave the oil, but I never let any of my other equipment run with oil that looks like that. BTW, it isnt using a drop of oil as far as I can tell. "
As far as warm ups...I let my 790 go about 10 minutes...or as long as it takes me to smoke a cigaret. This depends upon the ambient temperature, of course. This applies even if I'm just moving it out of the garage. If you don't let 'em warm up, you could get condensation in the exhaust system...maybe other systems as well. That might account for your white smoke, BTW. This tractor does have a temperature gauge...by the time 10 minutes have elapsed, the gauge is reading in the green. None of these machines seem to get to midrange on the gauge...
I've not had this tractor in the winter yet, but I'd always let my previous machine (a 670...very similar to the 790 except for engine displacement) idle at about 1500 RPM for about 15 minutes if it was below 40°F. I didn't normally run it too hard until the 15 minutes elapsed.
As far as the "dirtiness" of the oil...diesel engines will tend to "muddy" their oil quicker then a gasoline engine. Again..can't say I ever worried too much about it.
After the 50 hour oil change, I change the oil once a year. I don't put more then 70 or 80 hours on the tractor per year (normally less then that), so I think annnual changes are OK.
I expect this tractor to last me the rest of my life...so I will attend to maintenance.