Considering the price to maintain and heaven forbid, replace a large diesel engine, I decided sevral years ago to have my engine oil analyzed every year when I changed it. I realize this isn't for everybody, but it did help me one year when the analysis showed a high amount of contaminants. The lab tech suggested I examine the engine air filter for an element failure. Sure enough, even though the filter should have been good for another year, the element had torn and was allowing a small amount of unfiltered air into the engine.
Oops, I got off topic, sorry... During my first couple of years of engine oil testing, I discovered that Shell Rotella T motor oil retained it's viscosity better than Chevron Delo motor oil, so I started using Rotella T. Delo isnt bad oil, it just didn't hold up as long in my engine.
My youngest son works for a company that services forklifts. He specializes in large diesel units. Since his company provides a mobile service, he must keep his work van fully stocked. He called me the other day and told me that his company's motor oil vendor had excess diesel motor oil that they needed to get rid of. More specifically, the motor oil boxes had become greasy and dirty and were no longer "marketable." I thought this was funny, since they would get greasy and dirty riding around in the back of my son's work van anyway. So my son showed up at my door with six cases of Delo 15/40 motor oil. I looked at the brand name and said "oh." My son of course said, what's wrong? I told him I had switched brands, to which he replied, "are you kidding me? This is free dad, I told my vendor I'd take it...I have no use for it myself."
My son reminded me of the old saying of never looking a gift horse in the mouth. Considering the amount of money I will save on oil changes over the next three years, I guess I am temporarily switching back to Delo!
Sorry for the boring story...
Craig