Racing noobs lol Jk, Jk's. Mark 5 is a racing fuel made by Pro Racing fuels which has a Octane of 115 and it has a cool purple look to it The stuff is $100 for 4 gallons!. Mark 2 Might be alright in some of the higher compression flattys. I think the octane is 100. I dont know if you guys have ever smelt Mark 5 or any racing fuel but let me tell you it will clear your sinus-s. It has a wicked smell espeiclly when burnt!