For the people that are worring about the compatability of marvel mystery oil and synthetic oils, I went ahead and used my car as a guinea pig which is a toyota corolla 2000 model. I have done four oil changes to the car since I purchased it, the last two that I've done I used four quarts of mobil one synthetic oil (5W-30) and only four ounces of marvel mystery oil in the crankcase. If you do your oil changes at 3,000 miles like I do than that means that I have used this type of mixture for a total of 6000 miles. My car is still running strong and I haven't had any problems with it, not to mention that I am in a car club and we periodically go racing on Friday and Saturday nights which means that I drive my car very hard. I do have to admit that MMO is the only product that I have used, that you can feel the effects as soon as you pour it in, even if it is only four ounces. The effects are(smoother running engine, engine revs a lot quicker). Don't forget to also add four ounces in the gas at fill up for more miles per gallon.