Here's one for all you motorheads... Ever since I bought my new '99 XR600 3+ years ago, I have been religous about changing the oil at least every 4th ride (approx. 15-20 hours) with nothing but Silkolene semi-synthetic 10w-30 or 20w-50 depending on the time of year without incident... until now. I went to Moab 2 weekends ago. The day before I left, I put in a fresh batch of 20w-50 and a new Fram filter. By the end of the 1st days riding (6 hours or so), I noticed at least one of my valves was clicking. I didn't think too much of it because I adjusted the valves in January and maybe a locknut was coming loose. I pulled all 4 valve caps that night and sure enough my left exhaust valve was a little sloppy (however the locknut was still tight). I snugged it back down and took a quick sunset jaunt around Slickrock that evening. By the time I got back, it was clicking all over again so I had a beer, went to bed and dreamed of Poison Spider/Golden Spike/Rusty Nail on tap for the next day. First thing in the morning I pulled my valve caps again. Sure enough, the same valve was sloppy (the lock nut was still tight). So I snugged it down again and prepared to ride. In the 10 minutes it took to get to the trailhead it was right back to it's chattering little self. Ignoring my better judgement I proceeded to ride for the next 4-5 hours. While it performed essentially the same as always, the clicking never got any better... or worse. After returning home to Denver I took it to the shop. Of course their first question was "Have you been changing the oil?" After taking my valve cover of, they learned the engine was either not pumping enough oil to my top end or none at all which caused my cylinder head to melt and ruin all associated rockers, cams, etc. resulting in a $1,400 rebuilt top end estimate. He kept indirectly accusing me of running it out of oil, not changing the oil and/or using cheap oil because they could not explain why it stopped pumping oil to the top end. After repeatedly denying their finger-pointing, he mumbled something about "maybe it's the oil." Knowing I've been changing the oil regulary, I'm left with one burning question: is Silkolene to blame? For those of you that suffered through this long drawn out drama, any thoughts???