Re: 20 hours on diesel this summer. change oil or not?
I'm of a slightly different school of thought. I just changed my oil because it had passed the 50 hour new engine break in. It will have about 20 additional hours on it when I winterize. I'm not changing it again, instead I am going to take an oil sample and send it away for testing (about $20 with postage). I hope to get a better sense of real oil change intervals by the time the warranty runs out. Most modern oils have plenty additives to control PH, unlike the old days of SAE 30 or 40. I think changing the oil 'just because' is a bit wasteful nowadays.
That being said as the others have stated, it's hard to argue with the low cost of an oil change. It really isn't that expensive compared to a new engine.