Hertz, Avis and the other car rental agencies must have thousands of cars that are on certain maintenance schedules. When a car is returned they don't know if it was all stop and go, highway mileage, dusty conditions, short trip, etc. In other words, there is no way they can take any severe service into account - or do they? Maybe they always assume severe service just to play it safe. One thing is sure, they have to have cars that are reliable. I have heard stories of people buying used rental cars and they are given a very detailed maintenance record (so my assumption is they have to be doing it right) but I do not know what this record contains. Since these companies do not hold on to the vehicles as long as most individuals do, I am wondering if their service schedule is simply going with the OCIs (and other maintenance items) as recommended in the car's manual. I doubt they would use synthetics but I don't know. It seems to me that would extend the interval and save money in the long run. Perhaps they assume just the opposite and put the cheapest stuff in there since it would be hard to hurt the engine with whatever oil used before they rotate the car out of the fleet. Does anyone know how rental companies figure OCIs and the kind of oil they use?