Well I've always used castrol gtx oil.....from regular 10/30 on my old (now dead) vx engine to the castrol synthetic blend 5/30 on my zc the first 2 oil changes...now I noticed that it burned or used a bit of oil...like a quart in 5k miles....so I decided to use castrol high mileage oil...I'll post what results I get....it feels smoother on this 11 or 12 year motor I got.....it only has 45k on it but I though the additvies on the HM oil might condition the seals etc etc...