Certain conditions might require a richer mix, such as sand. The motor has to work much harder so it needs more oil. This is what I've been told, so maybe someone else with more experience could chime in.
that's incorrect....more oil will make it leaner.
KevinTwoStk and Dutchie have the right idea....go fatter jets/jetting.
I see. I must have remembered it wrong. Good to know though. I didn't realize it would lean out the jetting by adding more premix. That definitely makes sense.
I read about it over 30 years ago in an article Rick Sieman wrote in one of the Dirt Bike magazine issues...and like you, before reading that article I thought adding more oil would make it richer...back then I had a new 83 CR125 which took pre-mix at 20:1.
About 20 years ago, my buddy and I started talking about it one day, comparing each others opinions...then he decided to try a quick experiment.
He got two identical plastic cups, drilled a 1/16th" hole in the bottom of each one, then put an identical amount of fuel in each one....one had a 32:1 mix, and the other was straight gas.
The one with straight gas drained a few seconds faster every time.
There's a TON of info on the web about this controversial subject...even on this site...but you can do simple experiments right in the comfort of your own garage that don't lie.