Synthetic motor oil for 4 stroke automobiles is the consistency of a 10 weight oil... Considering you change to synthetics at 50,000 miles, you will have problems with oil leakage AND consumption... Synthetic is a LOT THINNER!!!! It will leak past the worn seals easier, and will get through the oil control rings easier!!! YOU HAVE TO start from new.... Synthetic oils are FAR SUPERIOR TO conventional oils!!!!! HANDS DOWN!!!! No question about it!!!! As far as starting from new with a 2 stroke motor,,,,It is preferred, but not necessary!!!! Yes, a 2 stroke motor will have worse compression now that it is worn, and the exhaust valves WILL BE significantly dirtier!!!! BUT,,, That is the lack of compression that is making them dirtier,,not the oil!!!!! A few posts ago, a guy complained of using KLOTZ and his valves were dirtier.... BELIEVE ME, If He would have started with a identical sled, started it off from NEW with KLOTZ, put it through the SAME torture,,,, the sled with KLOTZ would have 1/2 as dirty exhaust valves than the sled without KLOTZ!!!!! As a motor wears (significantly faster with yamalube) the volumetric efficiency goes down. As the efficiency goes down, the combustion residuals increase,,, thus "carbonizing the exhaust valves" (among other things) signficantly more than when it was NEW!!!! So, as it wears,,its gonna get dirtier.. THE TRICK IS NOT TO GET IT TO WEAR AS BAD!!!! Thats where synthetic oils come into play!!!! I WON'T USE AMSOIL!!!! Not because it doesn't work,,, quite the contrary,,,because of THEIR marketing strategy!!!! Its like their PIMPING it and letting ANYONE be a dealer!!!!! I use KLOTZ!!! Been usig it since the 1970's... IT WORKS FINE!!!!! Gary Oles nosboy