The only oil additive that made a huge difference to my motors was Bitron. One of the first things I noticed apart from a huge temp reduction on a hot day was the ache in the right ankle as I had to adjust my foot position and back off the pedal. Hard to believe but a fact. I used it for a number of years and then I could no longer get it. Another amazing example was the one for trans/diff and anything else you wanted to use it for, like drilling stainless steel. A mate of mine had a Falcon, XF maybe, and it would stall the motor when you were driving through a roundabout, foot off the accelerator and turning the power steering. All I had of that Bitron was about a teaspoon full, I put it in the power steering and bingo problem solved. Believe it or not
