I've got an old bc 1250 with the 80 horse Perkins in it. I try to change mine at 200 hours, but sometimes it goes over a little. 250 hours is the max it's been between oil changes. I do it myself because I'm a tightwad, and it also gives me a chance to give everything else a good look.
In my opinion, the biggest must do is keeping it greased. I give the disk bearings 1 shot of grease for every hour of run time, and do it 2 times a day.
Oh, and don't forget to keep the air filter clean, and the radiator washed out.
Andy