Yes my bike is a new 04 got it left over ..I used Mobil 1 synthetic every time so far .. The Owners manual suggests a range of oils between 10-30, 10-40, 10-50 and 20-50..... 10-30 is used when the weather temps are 14 degrees to 86 degrees. That covers MY Winter conditions to hot Summer really but in hot summer I could go to a 10-40 which will increase the protection from14 degrees to 104 degrees F. I would like to say that my kx100 has one of the nicest shifting transmissions I have ever felt on any bike .That was one thing I noted about the bike right away ..I change My oil about every 3rd ride. The bike only takes about 1/2 quart so its not alot of money .Hope this helps you out with your sons kx65.One other thing I would like to add,each container of oil has API ratings on the can .Im sure you have seem them . You should make sure you see one of these ratings SE,SF,SJ,SH or SJ with JASO MA on the can .Hope this helps..