Hey, guys, i had bought some pennzoil yellow bottle dino 5w30 some time back from walmart (4.73 litre jug for $10.00) and it came with a free bottle of pennzoil gas mileage improver. I'm just wondering what is this stuff? Is this a upper cylinder lubricant? solvent? injector cleaner? water remover? I haven't added any yet to my gas, but was thinking about doing so. Is it ok to add it after an oil change? Or is it better just to add it right before doing an oil change? Will it in any way contaminate the new oil? Please enlighten.