I understand changing it every 20 hours, but why not change the filter too. You are not getting the full benefit of a change. The filter is holding oil and you are mixing the clean with dirty and diluting your oil change.
I don't go by a set amount of hours. I watch the oil closely and when it's dirty, I change it. I do this with all my vehicles. To me it's like changing an air filter by counting miles and not looking at the condition. An air filter gets dirty from the air that goes through it. It could be dirty in 100 miles or 5,000 miles. It all depends on how much dirt goes through it and not a certain amount of miles. Oil is the same way. The oil gets dirty by a variety of factors including temperature, air quality, how rich the fuel mixture is, time, and others.
Changing the oil at factory recommended intervals is safe, IMHO changing it when it's actually dirty is better.