Hi Marsha, Many people mix brands of oil specified for either petrol engines and the same is true for oils specified for diesel engines if topping up, but it's not a good idea....It is a very bad idea to use an oil specified for one type of engine in the other. I would suggest never mixing oil manufactured for a diesel engine in a petrol engine or visa versa. The reason is that the diesel engine is dirtier than a petrol engine and manufactures of oil formulate it to have more cleaning chemicals than that used in petrol engines....Mixing the two can result in a build up of sludge which can actually block the lubrication system of the engine causing in some cases severe damage. Regards John