there was a FBO owner and flight instructor who used to break the engines in on 100LL, and then after one hundred hours, ran his "blend", which was two thirds mogas, and one third 100LL.
he found that running straight mogas was too clean for the engines, and freshly overhauled engines had particular problems with sticking valves. on the other hand, the 100LL was too dirty, leaving lead/carbon deposits.
his mix resulted in cleaner cylinders without valve problems.
essentially the mogas was too clean, and needed to be "contaminated" with 100LL. he could be rather unorthodox in his methods, but he was getting up to 4000 reliable hours out of trainer engines, so his methods had backing.
the only problem i found with mogas is difficult starting in cold weather
the big thing with mogas is DO NOT RUN ETHANOL blended gas. it really is that bad. it will attack older rubber (fuel lines and gaskets), it will also go after some softer metals like zinc and brass resulting in carburetor problems, in particular sticking floats. it absorbs water, preventing all the water from being drained out of prior to flight. this lowers the boiling point which increases the chance of vapor lock, and the water can drop out of solution with a temperature change.
