You can purchase regular lawn mower motor oil that is formulated for use in Lawn Mowers which is an easy way to know you're using correct engine oil in the lawn mowers engine.
However you can also use 10w30 motor oil that you use for vehicles in your lawn mowers engine and it will work the same as lawn mower engine oil.
I always use 10w30 engine oil in my lawn mower because I also use it in my vehicle and always have it available and it's cheaper too.