What is the oil fill hole in front of my gas tank on my 1991 Suzuki dr350 for

Your bike has motor oil stored in the frame instead of a " tank". there is a frame drain located at the lower portion of the front downtube of the frame and the engine. If you are changing the oil you should drain both the engine and the frame and then re-fill. since you bike has two pumps, one to feed the motor and one to push the oil back to the frame, you need to add some oil and then run the bike for 30 seconds or so. After you have run the bike for 30 seconds check the oil in the frame and add as needed.

thanks and I hope that this helps.
steve