Hmm, maybe there is an oil pick up issue with the oil in the low range? Sounds like a clogged oil gallery maybe, Silicone is great stuff to seal motors and valve covers, but if a small blob falls into the motor its the perfect stuff to find and clog an oil passage. Other than that, improper torque or seating of the cam retainers? Many times they have an arrow on them that must point forward, and they should not be interchanged with one another once the cam has seated from new. 200 to 300 miles sounds like a bit of a haul for a thumper dirt bike, but I have a Yamaha and need to change oil like every 150 miles. Hours are more important on a dirt bike IMHO, how many hours do you put on between oil changes.
I just changed the oil in my WR for the first time this weekend since switching to Mobil 1 full syn oil, 15/50 wt. I noticed almost NO metal shavings in the oil filter as compared to previous oil changes. I almost didnt have to clean the oil filter it was so clean!! I had neglected my oil change an extra few hours too so Im pretty sold on that stuff, its a bit expensive, but Ill put the $$$ into my oil if I belive it will keep the motor together longer.