A few comments regarding Sherco main bearings:
I replaced the main bearings in my '03 290 in April, 2007. My problem was a worn left-side bearing. The right-side looked OK, but I replaced the bearings and crank seals on both sides. There was no rust on either bearing. Actually, my issue was wear and heat-related damage - pretty much the opposite end of the spectrum from the rust issue. I feel certain that the bike may have experienced a rough childhood before I got it. Most likely a diet that contained a lot of dust and lack of proper air filter maintenance.
I ordered my bearings from Ryan Young Products using the part number from the '03 Sherco parts list. When I received the bearings I noticed that they were sealed on both sides. I immediately called Ryan to make sure I had not received the incorrect bearings. He assured me that I had received the correct parts and that the revised bearings were supposed to be sealed. This seemed rather odd to me even after our conversation, so I sent him an email to get written clarification. He replied with the same answer. "The new bearings are sealed. Leave the seals in place when you install them." I also discussed the sealed bearing issue with Brad Baumert at TTC this spring. Apparently, some engineer at the Sherco factory had explained the sealed bearing reasoning to the RYP guys and was confident that this was the way to go.
I am happy to report that these sealed bearings finished out the '07 season and have performed flawlessly through the '08 season as well. They still seem just as fresh as they did when I installed them. The sealed bearing concept seems odd, but I am now a believer.
For what it is worth, I run premium unleaded with Maxima K2 @ 80:1