The crank bearings, rod bearing, wrist pin bearing and cylinder all need proper lubrication. On bikes that run premix the fuel charge goes through the crankcase on it's way to the intake ports and takes care of that task. Holes are drilled in the centercases accordingly to ensure that the crank bearings come in contact with the fuel mixture and get lubricated. Oil injected bikes typically have oil lines going directly to the crank bearings, that oil is then carried out of the crankcase into the cylinder. When switching from oil injected to premix you have to verify that the crankcases are configured (drilled) to allow the fuel mixture to contact and oil the main bearings. If this isn't done there is a risk of loosing the whole bottom end.