transmission ?

[Hunted1]

I bought this one used with 5k on it. Has 20k on it now. Warranty just died and I figured I would extend it. The tow canned you need to run 87 octane though?
I've only used 85 in this since I bought it. Is it OK to use 85 octane with a tow tune or would you recommend 87?
I know stupid ? but the manual says 87 octane. The 87 is cheaper here by like 10 cents. I just don't want to mess anything up. I bet you have heard it all by now.

Thanks for all your answers. I figure that you never learn until you ask or read everything you can on the subject.