If your not using premium in the SVT what are you using? I wouldn't suggest prolonged use of lower octane fuel.
Umm, Kerosene!
Kidding.
As a skinflint, I try and keep the octane in the tank at around 90-91, premium level.
So, what I'll do is run a full tank of premium to about 1/4, then top it off with 89 octane mid-grade.
Then, once that tank is empty, I refill with premium and repeat the same process.
Occasionally, I'll even run 89 octane straight up on highway runs.
No change in performance that I've noticed, and it saves a few bucks.
BTW, Big Jim of California claims to always run 87 octane without any problems in his EO, so I think I'll be fine.
