Per my Rotax training at LEAF:
If you are only using auto fuel, you can use any "motorcycle" 10w-30 oil. My choice is Mobile Racing 4T. A synthetic like this is safe to easily make 75 hours or more with regular use. If you are flying with mineral oil and auto fuel, 50 hours is a safe mark. The only time you would need to change more frequently is if you are having a steady diet of 100LL (this would also require the use of a semi-synthetic like the Sport +4 or mineral oil).