I would, I've contacted dover about oil, other than what they use he said to stay away from motor oils. the only oil I would use from a auto store is royal purple. if your using cool power I mix it with power plus a 50/50 mix works great.
Amen on the 50/50 mix. Power plus is hard to beat and mixes with cool power very well. A great way to hold the cost down a little. Definitely flush the crankcase. It would be even better if you pulled the crankcase cover and flushed every thing out with carburetor cleaner. You're also going to see a little power gain. Good luck!!!