Read
Rotax SI-18-1997 for more information. It seems the lead from Avgas (contained in the blow by gases) accumulates in the engine oil and leeds to problems with the gears. Additionally the scavengers added to Avgas promote corrosion.
So : If you don´t fly many hours a year, mogas will ´gum´ in your tank and in your carbs, causing problems. Avgas cause corrosion to your engine´s internal parts, if you don´t fly much.
Only sollution : fly more
Anyway it seems to be important to use the right oil with the gas you normally fly, if you fly on Avgas don´t use fully synthetic oil (for corrosion reasons) and use oil with gear additives and change oil more often.