The bikes use forged pistons, let it warm up throughly before riding. Heat cycle the engine a few times, then ride easy, let it cool, and repeat. Get a tank of fuel through it before really reving it out under load. The jetting will probably be rich, leave it that way for break in, then adjust as neccessary. Done this way the top end should last a season easy with clean filters and good oil. I run Amsoil 2000 @ 40:1, but Motul, K2 are good as well. The GasGas clutch/transmission is kind of finicky about oil. Spectro 80wt. gives the best performance in these bikes. Mine came with what looked like ATF (very thin). I drained it immediately and refilled with the Spectro. So far (about a year) this is the smoothest, most durable 2-stroke motor I've ever had. Linkage is well greased and sealed well.
A few heads up items: Check that the rear pipe mount bolt is not touching the fuel tank. I made a plastic insert to prevent this. Route the trans vent and a pair of carb vents to the airbox if hit any water crossings. The seat bolts don't line up with the insert nuts in the seat mounts unless the seat is forced down and foward slightly. Like any new bike, antiseize and locktite bolts and check wire routing for possible chafing problems.
The bike may be quite stiff before the suspension breaks in. Give it about 100-200 miles. I think WER consulted on the '01 valving, so it should be good for the woods. Good Luck!
Glenn
'00 GasGas XC250