I'm going to add 3 gallons of Toluene to roughly 10 gallons of 93 pump gas (or whatever it takes to fill it) in my '05 STi.

My question is how much "Marvel Mystery Oil" should I add?

Is there a recommended ammount per gallon of toulene?
Is it even necessary to add the oil?

I know some people will add straight transmission oil, but I already have a bottle of MMO.

Thanks.

It's necessary to condition the rubber fuel lines. Toluene is a strong solvent which can/will dry out the rubber. I'm not sure how much, it's been said to add "afew ounces", but i have no idea how much that really constitutes... :)