Remove the mower deck.
Slide under the tractor.
Directly underneath the checker plate floor is the travel/lift valve.
Clean off the underside of that casting and find the hex head plug. This is a female hex that faces the floor. You must clean the opening out thoroughly so that you can tap an Allen Key into it, all the way. The best Allen Key is the one that works with 3/8" drive socket-wrench sets. You get ONE CHANCE to crack this plug loose so buy a new Allen Key socket and make sure it is seated all the way in before attempting to remove this plug.
With a drain pan under the tractor, finish removing the plug.
You can take out the spark plug from the engine and then spin the engine over for 10 seconds at a time until you notice that the oil begins to spit out instead of coming out in a solid stream. The spitting indicates that the reservoir is now empty. Stop the procedure and put the drain plug back into the valve body and snug it up.
One of the easiest to find oils is Shell Rotella T 15W40 MOTOR OIL but you can also use any well-known brand such as DELO. An alternative is 20W50 MOTOR OIL, especially if you are living in a temperate climate where it rarely gets colder than 32 F.
This can be a messy job so laying a plastic sheet on the garage floor is a good idea. You should buy 7 quarts so that you have some oil left over. If your tractor has the reservoir under the battery, then it is full when the oil level is 6 inches below the top of the reservoir fill neck tube. If you have an older 220 with the front mounted tank, then fill it to about 1 inch below the actual top of the tank, not the fill opening.
Oil expands when it gets hot so you need some headroom in the reservoir or you will end up with oil seeping out the vent hole in the cap. Install the spark plug. Start the engine. Cycle the mid-lift cylinder up and down a full times. Drive the tractor around in forward and reverse. Shut off the engine. Check the oil level and top up if necessary.
Change this oil every 2nd year whether it looks OK to your eye or not. There is no oil filter and your eyes cannot see contaminants.