As usual LD1 puts in some key points - "extended oil changes" and "regular maintenance". Older equipment recommended oil changes based on the oils and filters of their time with a healthy margin for CYA so they didn't have warranty issues. Newer equipment also has tighter tolerances. So I need a UOA (Used Oil Analysis) to establish a pattern.
If you only "use a lot of gallons of oil a year" you really need to sort out your equipment and determine where your oil dollar is going with appropriate testing.
For example I've a number of small 4 cylinder engines (lawnmowers, generators) that only use a quart or two and are real easy to change, don't have oil filters. A $20 test on any of these would cover many years of oil changes. Then there is my F350 which takes about 4 gallons and a filter and was recommended to have the oil changed every 3,000 miles under heavy use. Which would be 5 changes per year at about $100/change if I DIM.
By testing I found with my usage I can expect to go about 20,000 between changes using T6 in my F350 with excellent UOA's, so I went from 5 changes per year to 1.
So if skipping an oil change would save you the cost of several UOA's a UOA is worth it. But if it's not then why worry.
All valid points to some degree. The tighter tolerance thing, even the tightest tolerance of any modern engine are at least 4-5 microns. Any motor oil, even a conventional, is considerably smaller than that. Many bypass filtration units will filter down to the 1-2 micron level, and oil goes thru them just fine. The tighter tolerances argument seems more of an internet related folklore or marketing thing to convince people to use lighter weight oils. That comes from a second misconception, that oil weight is a thickness. It is not. Oil viscosity is a measure of resistance to flow, not oil "thickness". A motor oil is just as "thick" at 250F as it is at -50F. Just the resistance to flow is different.
On the oil testing side, I am a firm believer in UOA's for my commercial stuff for the same reasoning you present... to keep oil costs as low as possible. But while the oil may still show to be good via the testing, there is more to that. Tracking trends and patterns in oil sampling, your engine can give you clues as to whether there is a conflict with higher OCI's. For instance, for my Detroit 12.7L, the OEM recommended drain interval is 15,000 miles / 300 hrs. I have taken my oil, via testing to 22,500 / 450 hours. The oil still shows being serviceable at that point, but thru numerous UOA's the patterns show that at that point, accelerated wear metal rates and other factors are deviating from the gradual trend line and pattern up to that point. There is a gradual, normal increase in wear metal concentration as the oil is used, but at a certain point, the rates start of deviate from that pattern. The engine, in it's own sweet way, is letting me know that, while I can extend the drain 50% from what the OEM stated, there is a point where any longer is detrimental. Yet, per the UOA lab, the oil still has considerable life left.
You have to keep in mind, a used oil sample only tests the oil. It doesn't test the engine condition. You have to watch trend lines and patterns to see the clues from the engine itself. Oil sampling analysis is as much a art form as it is a science.
