Some of AMSOIL oils can go 25,000 miles or 1 yr. providing you use their oil and air filters. They have been making that claim since 1973, see pic below. I have done it and I have customers who are doing it.
There are lots of myths about synthetic oils. Believe me, I have used nothing but synthetic oils since 1975, sold them since 1978 and to this day these rumors persist to this day that were started back in the 70's.
There is no truth to the myth that once you use synthetic you can never use anything else.
There is no truth to the myth that seals will start leaking if you use synthetic oil. Seals leak for various reasons and its rarely linked to oil.
There is no truth to the rumor you cannot use synthetic oil in a new engine. ALL POLARIS ATV'S AND SXS COME WITH SYNTHETIC OIL FROM POLARIS. All Corvette's come with synthetic oil as do all Cadillac's as well as many other makes and models. If you want to best, then synthetic it is.
Step back in time before many of you were born.
Popular Science 1976....Would you believe back then even Mobil 1 was saying 25,000 miles? True, right on the can.