I believe this is a direct response from Amsoil.
"In order to obtain the highest level of protection, the AMSOIL Dominator Racing oil contains what is referred to as ash-type chemistry. Though this type of chemistry provides the additional safety margin required in racing it also tends to allow for additional deposit formation within the combustion chamber. The rate of formation is slow but over the course of many seasons of operation, it could present a problem in certain PWC applications. In competition, engines are inspected or rebuilt frequently. That being the case, ash-type chemistry and hence deposit formation is never really a concern. In an application in which the equipment may run for years without inspection or cleaning, the possibility of a concern does exist. Hence, the reason for our warning statement you noted. If you have performed some serious engine modifications, the AMSOIL Dominator would be the preferred product. You would just have to remember to inspect it for combustion chamber deposits every few years. I hope this helps."