I have a 1999 Ford F250 - 7.3L Diesel 4x4 with 56000 miles on it. Last time I changed the oil & filters was back in 2017. I've since put on 600 miles on that oil change to date. My question is does the oil go bad sitting in the crankcase that long? It's awfully expensive to change the oil if it's not needed. 7.3L Diesel (15 quarts of oil). Bob
Well it does go bad over time they tell you to change it once a year. If you don't use it so it's been sitting that long I'd definitely change it
The basic rule of thumb is at least once a year. At the same time, oil could theoretically still be within tolerance even after that period.
In theory, you could test your oil with an oil test kit, like Blackstone Labs https://www.blackstone-labs.com to see if it the oil is still good.
I usually change oil at least once a year, just to be on the safe side of things. But my oil changes aren’t so expensive!
I suppose if any engine could go 6 years without an oil change, it would be a 7.3 Powerstroke. Very durable motor.
That said, those things are very desirable and worth maintaining as long as possible. I would definitely do it once a year regardless of miles. But since you are changing so infrequently, it would be OK to get the cheap oil from Walmart (or Amazon, or Costco), or see what oil Rock Auto has (I buy oil from them all the time, and it is typically 15-20% cheaper than retail).
Quick price check: if you go with Walmart's house brand conventional oil, should be able to do it yourself all-in for under 50 bucks.