On the Toro the tolorences are close. Thus 50:1 is all the oil that is needed. On a cheeper engine like a Weedeater they are not as close, so therefore 40:1 and on alot of the cheeply made pocket bikes and chineese chainsaws the ration is 25:1. They also meet EPA standards but the tolorences are not as close so they need more oil to run well. Just like in the 70's 100,000 miles was alot on a car engine. Moset didn't make it that long. Now with closer tolerences in machining they go much longer on avarage. If you run an engine that should have 50:1 at a higher oil content all you will do is cause excess carbon to build up which will lead to premature wear and overheating.