Originally Posted by Maximo

its the fuels...some fuels are too thin to create a proper seal in the engine.......Novarossi recommends 12% castor oil for a very good reason.......some fuels can actually make the proper seal, others will not.... fuels with thin synthetic oils struggle more with creating a seal then fuels that use castor oil

Novarossi engines have a larger diameter vacuum return port drilled into the case...I believe this port is there to return excess fuel and oil that accumulates behind the front bearing to prevent leaks...However if the fuel is too thin this return port will vacuum the oil seal ring dry....allowing the engine to breath thru the front bearing

here is the vacuum port



and here is what can happen if running on a dirty dusty track





And as I say the problem is deeper then the front bearing, as these bearings do not create a seal, these engines rely on fuel/oil film along the crank to create a hydraulic seal....if the fuel is too thin it will not seal and the engine will breath thru the bearing.......

I am interested to see some people do this test, report back their findings and what fuels they are using.......