This is one application where synthetics are vastly superior, quality synthetics anyway. I've been involved through the years continually upgrading an even more severe application. Synthetics can handle more heat than mineral oils, the right ones have superior film strength, and are just darn good. Kubota sells what I am sure is a good mineral oil in its 80W-90, but I think that is to be cost competitive. Remember that when a manufacturer sell an oil so many companies up the supply chain have taken their piece of the action it makes it hard to compete. If the mineral oil will do the job for 95% of the users, supply that. Have the dealer put the super oil in the hands of the customer they know will run to the limits even though that super oil may not be the manufacturer's brand.