I have oil leaking onto the left side tire of my pronto m71 sure step invacare chair. why? thanks for your help.

Sounds like you have a blown seal on your left gearbox. The only permanent solution is to replace the left brake/motor gearbox assembly. Not exactly a do-it-yourself repair. to verify, you can pull the seat off to inspect with a flashlight or tip the chair up at 45 degrees on the right drive wheel to inspect.
In my experience this is a common problem. It may take some time for enough gearbox grease to ooze out and cause the gearbox to fail. If you run the chair inside the house it will get messy very quickly and the grease stains are difficult to remove. If the chair is upwards of 4 or 5 years consider replacing left and right motor/gearbox assemblies. Hope this helps.