When to change your oil?

If I were to post the question of how often to change your oil I would no doubt get 100 different answers. Most owners’ manuals state something like every 6,000 miles or 6 months; whichever comes first. That said, the proverbial little old lady who drives her car once a week back and forth to the liquor store should change her oil even though she has only driven 1,000 miles in 6 months time. But why; does something happen to the oil while it is sitting in the oil pan? When I owned a motor boat with an inboard V8 engine I had the oil changed in September when the boat was winterized. Then 6-8 months later, I put the boat in the water and began running the motor. Although this is more or less a standard maintenance practice for boats in cold climates it seems to contradict the mileage vs. time oil change interval recommended by the auto manufacturers. Does the oil in your oil pan deteriorate, accumulate moisture or break down over time? How is oil sitting on a shelf in the original container different from oil sitting in an oil pan? These questions came to mind while I was trying to decide if I should change the oil in the motorhome. The oil was changed one year ago and has less than 1500 miles on it. With the motorhome requiring about 25 quarts of oil at a cost of more than $200 for an oil change I didn’t want to guess about changing my oil. After a bit of research I discovered the concept of having your engine oil tested. This involves taking a small sample from your engine and having it tested at a lab. The lab will send you a report indicating your oil properties and the elements found in your oil. They also provide their opinion on the overall operating condition of your engine. The cost for this is $32.50. Bottom line: my oil was as good as new and they recommend I run it for 7,000 miles more before testing again. They also said my engine was in excellent condition. But what if I had gotten bad news, like fuel in the oil? An oil report can also alert you to a problem so it can be corrected. What about really bad news? Maybe it’s time to buy an extended warranty or think about trading it in?

Oil report for our Caterpillar diesel engine. Click to enlarge

A hand pump is used to withdrawal an oil sample out the dipstick tube. The sample bottle screws right onto the pump.