I very good question, but I don’t have a great answer.
I have been repairing cars professionally for over 30 years, and I read a few trade journals each month. Oddly, I have never seen an article clearly stating when your oil should be changed.
Valvoline, Mobile, and the other manufacturers clearly need to test their oil, and the car manufacturers must also test the oils used in their cars. Unfortunately no one has ever released the results of their testing. I have spoken to my oil reps, and they don’t have any test results they can give me.
Years ago Valvoline introduced a new oil for “High Mileage Cars.” I told the rep I wanted to review the data showing me that this “NEW” oil really was better than their usual oil. He did not have access to this data, later he called and told me that Valvoline did not release any test results to prove that the new oil is any better than their usual oil.
If this High Mileage Oil was better how come Valvoline does not release their test results to prove their claim? They will charge more for this oil, and maybe it is better. I don’t know if this oil is any better, and as far as I know, they have never released any proof of their claim.
Getting back to my first question, how often should you change your oil?
Due to the lack of real data/proof, I fall back on the old adage:
1. If your car uses conventional oil you should change it every 3 months or 3,000 miles whichever comes first.
2. If you use synthetic oil you should change it every 6 months or every 6,000 miles whichever comes first.
I have owned many cars over the last 30 years- mine and loaners. I have always changed the oil in these cars every 3 months/3000 miles, and I have never had any engine problems NONE!
Also, by bringing your car to me on a regular basis, you will know of a small problem before it becomes a big and expensive repair.