Cadillac Repair: Engine oil, mineral oil, crankcase

Question
When the crankcase has been filled with 10W30 oil,is it safe to later add another quart of a different weight, like 10W5?  Also, can different brands be mixed?  Thanks.

Answer
Hi Roy,

     Adding 1 quart of a different weight should be ok with the weights that you gave because it will get diluted with the other oil that is in the engine already. Putting the next weight or two up should be ok. Your owners manual will give weight recommendations depending upon how hot the outside temperature is and what type of driving you plan on doing. If you have a new vehicle and it calls for 0W20 or 5W20 then putting in 20W50 might be a problem because it is very thick and might not be able to get through the bearings and pump properly.

     Mixing different brands of oil shouldn't be a problem. However if you are using a synthetic oil don't add 1 qt of regular/mineral oil to top off. By the same token if you are using mineral oil don't add a synthetic.

     For some added info the first number means how the oil will flow at 0 degrees and the second at operating temperatures. So in the winter you need an oil with a low number and in the summer you need a high number so the oil will stay on the bearings and lubricate so the engine doesn't make knocking noises.

     Hope that answers your question. Bill