Chris142 you are, or were, over on BITOG... the fountain of knowledge on oil. What would they tell you over there? Oil analysis, right?
The oil is good until it's not and modern oil usually lasts longer than people think, but in reality anything said that isn't based on oil analysis is just a lot of jaw-jacking and wild-assed guessing. The life of oil, long or short, is an equation made up of the driving scenario, the type and condition of the engine and the oil and the oil filter used. That's the equation the OE uses still and back when your truck, and ours, were built. They factored in what they knew AT THE TIME, gave themselves a very good safety factor and set the interval. Remember it was done with 1987 oil, which is not nearly as good as todays choices. We have much cleaner fuel now too, which is an oil life extender. On the downside, our engines are now OLD, and that's an important factor.
If you want an objective way to evaluate the OCI FOR YOUR INDIVIDUAL SENARIO, run your normal 4K, send a sample off to Blackstone Labs from what you drain (or you could take a sample without draining if you have the means to do so), making sure you have the TBN tested. If the oil tests good, TBN of 1-2, low insols, small fuel percentage, etc., add a thousand miles or two and repeat until you reach the mileage when you see any of the indicators start to go south. Back off a little for a safety factor and that's then your limit and no more tests are necessary unless you radically change some part of the equation. Anything else is just a guess. If you look on the diesel oil analysis section at BITOG, you will observe that oils can last a very long time in service. Usually it's longer than most people guess, or the standard OCI, but sometimes it's shorter or right at the interval. Ain't no one size fits all. You aren't hurting your engine by changing the oil early, but you aren't helping it either. Mostly, you are just wasting your money.
Also, adding oil, as with a leak, is an oil life extender because you are frequently refreshing the additive package. The oil filter is keeping the oil clean at whatever level it can and also remember that oil filter efficiency improves as the oil filter ages and loads up.
In my own IDI scenario, historically, once I started checking I found the oil in my truck was going very strong at 4-5K with conventional oil and either the standard IDI filter or the PSD filter (I had long been doing 4K intervals). I extended it to 6K and was testing that interval when the heads blew off. ( : < ). After the overhaul, I put in 10W30 Triple Protection Rotella conventional (installed with a break in additive) with a bypass filtration system in addition to a high efficiency primary filter, both of which are oil life extenders, so I am headed for 8K miles until the next test. That will be a four-plus year run it looks like. I have taken two samples in between and both tests were good. I had particle counts done too because I was concerned about break in metals... but, no surprise, a 3 micron bypass system really keeps the oil clean so a break-in change wasn't needed.
