My 2005 started leaking at about 5500 miles, same time I switched to synthetic 10-40, it was leaking pretty good that I had to wipe it every day. At 6500 miles I went back to dino 20-50 and the leak stopped, now at 7200 and still no leak. I'm not saying it is the oil but that is my observation at thIs time. I can not remember off the top of my head what brand of oil, but it is that popular one sold at Wal Mart. Like RT 4 or 4T something like that.
Great insight. Things I've read about synthetic tends to backup your observation.
On a recent thread, after the base gasket started leaking a member of this forum checked and found head and cylinder bolts loose. He re-torqued them and the leak went away. On another thread the same thing happened, but the guy over-torqued just a couple of the bolts and it made the leak worse. So, if I were to get another base gasket leak I think I would re-torque to specification all the head bolts using the head bolt torque pattern to ensure even pressure on the head and cylinder. I would also re-torque to specification the bolts at the base of the cylinder that hold the clutch cable mount.
If, after re-torquing the fasteners it continued to leak, I would replace the base gasket and associated o-rings. If you have any mechanical experience at all It's not that hard or time consuming. With no experience I believe it's still do-able by following directions and maybe getting a little help.
I respect other's experiences and opinions, but my experience has been that the version of base gasket does make a difference. The first time I did my base gasket the parts person led me to believe the newer part number for the base gasket would supersede the older part number. I was in a hurry (first mistake) and didn't confirm by part number that I had the newer style base gasket. I know, pretty dumb of me. I did replace all the o-rings at that time. In less than 2,000 miles it was leaking again. On the second time around I simply ordered all the parts for a 2010 TW. With about 1200 miles on it now, it's as dry as a bone.
With such a variety of experiences it's hard to nail down exactly what's going on. We know from the o-rings that the oil feed to the head is around the right rear head bolt. Since the oil is under pressure there you would think that would be where the leak occurs. But I've never seen or heard of the leaks occurring there. In my experience it's always been on the left side of the cylinder around the timing chain cavity.
I don't remember seeing an oil flow chart, but believe at least some of the oil from the head returns through the timing chain cavity to keep the timing chain lubricated. I believe the action of the chain spinning flips the oil around some and this is what causes it to migrate through the most poorly sealed area of the crankcase/cylinder junction. The timing case is also open to the spinning crank and gears. As others have pointed out, other areas of the cylinder are well sealed due to the use of o-rings. The addition of beads of sealant on the newer version of the base gasket tends to act as an o-ring to better seal the area around the timing case.
Well, as you can see there is a lot of supposition going on in the 2 previous paragraphs, but it's my theory and I'm stickin' to it
, at least until you guys correct me.