My dad has a 98 Taurus with the old 3 liter Vulcan that he tells me is starting to rattle a bit before it's warmed up and the gas is pressed while it's in gear. He put a tank of 91 octane fuel plus a bottle of STP fuel cleaner in it and he says he notices a bit of a difference. It isn't an engine rattle as he held the engine at about 5500 rpm in neutral with the hood open, and drove in 1st gear at about 5500, and we couldn't detect any rattle. I was thinking maybe there's some carbon buildup on the pistons or valves? I then thought to myself that maybe that MMO/ATF mix that's been discussed on here might help his engine. What would the proper blend be for a 16-17 gallon tank of fuel?