Mowers are frequently running in dusty environments. The most important factor, in my mind, is getting that oil out of there when it's dirty. Note that the quality of the original oil comes to lesser importance in this situation. That is, change it at 25 or 50 hours based on dirt and pay attention to the air filter.
Having said all that, I find today's dino oils to be so expensive, there is little difference in price to use synthetic.