What type oil should you use for a 13hp generator?

Since your generator must be ready for use in any weather, Mobile One 0w40 is an Excellent choice. Try to change every 50 hrs if no filter, 100 with filter. However, you can push to 100 hrs to get you through the outage using the synthetic 0w40 without a filter...keep an eye on the level though if it is a fairly recent machine, made in the last 10-15 years, it may have a low oil shutdown. Do not run over 100 hrs, and only to get you through a storm...50 hrs is recommended.

Use the exact weight of oil recommended by the manufacture of your generator and nothing else which may not be 0w40 weight or may be. I do recommend you use Synthetic oil in any air cooled engine. Any major brand will provide great protection.