I have a 1990 115 Merc. which has been a great motor and will have to continue to be and I am due to add oil to the injection tank and wondered if anybody has switched to a full synthetic. I have used the Mercury brand oil (non-synthetic) and had no issues. My last fill was a semi-synthetic brand for "older" motors and has been the smokiest, worst oil I've used. I'm thinking of using Amsoil HP injector since it's easy to get and fairly reasonable in price. I have used synthetic in my weed whacker and chainsaws and seen dramatic improvement in performance and a lot less smoke.