It's too late to change so keep it and put it on a seasonal site so you use it more. Cut your costs by decreasing the insured value to $90K or less which should decrease your excise tax too. There is no good reason to do an annual oil change. At 18 years old it will be in better shape than most 10 year old 5er or TT.
I'm at 10 years on my Monaco and see no reason why it will not go 15 more without major expense. I waited until I could use this expensive toy 100+ days/year but that's too late so do not worry about it as it's PAID For.