Re: Oil filter size....
Fill the oil until it raches the "FULL" line on the dipstick while the tractor is parked on a level surface. Don't be measuring out "X" number of ounces thinking that is exactly what is required. As you noted, the filters can vary in size depending on who made them etc. Best to have the level of the oil at the right spot (as per the dipstick) for the pick up tube to always be under even if on a moderate slope. Even in my cars and trucks, I fill to the dipstick and then run the engine a few moments, shut it down, wait a fer moments for the oil to settle and check again to see if I need to add to compensate for the empty filter that is now has oil in it. I would also do that for your tractor.
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ALWAYS remember to remove the spark plug wire from the plug before sticking your hand in the machine!!!! The fingers you save could be your own.