ORIGINAL: NJRCFLYER2
1. Change both bearings. Normally the rear bearing is the culprit, but you may as well do both. Best bet is a stainless steel replacement for the main bearing, or a rubber sealed bearing as a 2nd choice. Rubber sealed will eventually wash out the grease inside and will generally not last as long as the SS. If using rubber sealed, best bet for an after run oil is plain old castor, to avoid damaging the seal. Some after run oils may damage the seal. Do not replace with the standard factory main bearing, unless you like changing bearings often.
Personally, I don't run bearings with both seals fitted. Did that a couple of times and had short bearing life, even with a Stainless. I think the main bearing may run too hot without the cooling effect of the fuel/oil vapour flowing through it . . my opinion only.
I remove both seals on the main, and the leave the external seal in the front bearing (removing the seal facing in for lubrication).
PS. l also had a main bearing seal come loose sending "seal shavings" through the engine.
Cheers, JB