I worked in an auto shop in high school in mid 1980's. Back then the type of engines that'd come in that didn't have oil changes often enough was an entirely different world... Many times I'd pull a valve cover and see the everything inside covered in a 1/2 inch thick layer of sludge, and you really couldn't do much about cleaning it because the whole engine inside and out looked like that. But these days, the amount of sludge you get is way less severe. Engines run so much cleaner these days. And my Prius with 300K miles is still hard to read the dipstick after an oil change because the oil is clean and stays clean for a long time. As in, not changing your oil often enough is way less of an issue than it used to be.
The tolerances were much wider too. Clean out the sludge, you'll likely get oil leaks or run into problems that were not there before you cleaned it out. Most engine covers could be safely cleaned without messing up the rest of the motor. Of course you tell the owner to change the oil more often to avoid these steep bills. It was really funny back then watching an engine guy trying to rebuild a transmission. They could get away with the dirty mess they were leaving in a motor rebuild; but NOT in a transmission rebuild.
Water is the enemy of engine life, though a couple rungs down from the biggest ones -- like not changing at correct intervals, or not maintaining proper level (going above, esp). In humid climates, lots of water vapour gets sucked in with filtered air. This vapour finds its way past the rings into the crankcase (esp cold), then condenses and pools at the bottom of the pan, creating acid which etches metal surfaces. In conventional ICEs, their continuous operation means 10 mins of running would heat the oil hot enough in most climates, to turn that water to steam and escape out the exhaust... but in hybrids there's so much time not running, doesn't happen often enough -- which is why driving a Prius requires the ICE kick on a fair bit, just to rid itself of moisture -- another reason I prefer Normal over Eco mode. The fuel issue, though: in other GDI motors the lack of extra injectors to run in a port injection mode, means fuel at 300 - 3000psi is being shot into your cylinders -- and past your rings -- every compression stroke. This also collects in the crankcase and dilutes engine oil. Toyotas have D-4S systems w/ port injectors to share duties, so can lessen the effects. But esp if you take your pedal to the wood in your 5th-gen... you're going to want to change that oil / filter often. I change at my old ICE intervals -- every ~3500 mis. This is roughly twice between dealer oil chgs at the normal interval. It's humid in HI... and the job isn't super-cheap but still affordable to do myself (waaay easier than the last car, for sure). Plus, if I want to save a bit and skip that 2nd in-between change, it's still twice as often as the factory interval -- which imo is excessive even for a hybrid. No 0W-16 synthetic can handle high humidity and DI modes under heavy throttle diluting the oil, for 12 mos... and not compromise oil film integrity.
My GM's OCI is 7500miles. All four of them. I've been changing it at somewhere in the 3000-4000 range. Oil tests are more expensive than my oil changes....but I cheat. I use Super Tech, since it's Dexos rated and <$20 for a 5 pound jug. OEM Filters are $6 or less. Labour is free, and I know who to blame for mistakes (over/under torque, over/under fill, not changing the filter.....etc.)
Go over to bobistheoilguy to see the debates over oils, filters and intervals. None of us have the comparatives to be able to judge, certainly I don't. On a new car I do a 1, 5 and 5 thereafter interval without being super fanatical if it is 4600 or 5400. I use the oil the engineers at the manufacturer specify. I don't bother with UOAs. I keep my cars for 6-8 years and sell with less than 50k miles but with a well documented history of maintenance. My cars sell for more than KBB and always within 4 hours for cash.
@mikefocke : that's prudent and quite responsible, incl'g for the next owner. My first 3 cars were all used, in varying degrees of decrepitude... but the Paseo I owned from '96 - '12 (bought w/ 51K and sold w/ 266K). Still had its OEM clutch, steering rack, and bearings. Toyota doesn't make cars like that anymore... because cars that simple and overbuilt have been either legislated or cost-benefitted out of existence. Next two cars were used heaps (man that 7th-gen Civic was a pile)... and my first new car, was a 10th-gen Civic (last Honda I'll buy new). Traded in paid-off at 5y, for the 5th-gen Prius. Great you were able to turn your cars over so soon and so often. But my current gig is very, very seasonal, esp since post-covid... so usually own cars past the note before trading in -- the 5th-gen Prius once researched, was a shoe-in, no competition. Plus were a moto mechanic 16y... so would say yes, I do have comparable data for oil-related wear re: intervals (from all manner of customer powersport vehicles, and the bevy of lame excuses the great majority have for hard parts expiring 'sooner than expected' -- if you wheelie your ZX-9R out of the parking lot and continue to treat it like a unicycle for 50K miles... why wonder about its ugly rod knock? ). Your use case doesn't provide much challenge to even ask the question of oil-related wear and intervals, since that would require well, brinksmanship which you obviously don't practice. But those of us without the resources you take for granted to care for your vehicle, certainly do... so would be interesting to get owners in similar life situations, to log all events and mileage, incl'g climate / weather data, type of fuel used, DIY svc'g, etc. Then we'd have actual use case data rather than just bench tests and lab results. Damage is damage is damage... also easy to spot the various kinds (oil film breakdown from too-low-level, water in crankcase oil, fuel dilution -- all possible at the same time) in an ICE
Everything affects everything. - and generally speaking, you will not go far wrong by using the manufacturer recommendations. Even in @mikefocke 's use case - HE would almost certainly would the same results were he to drive Priuses and use the 10K OCI, because most of the lubrication related failures in these cars do not express themselves until you're well into the back 9. HOWEVER (comma!) if you buy a Prius even NOW and want to keep it for 15 years and 250,000-300,000 miles you "may" not want to stick with the manufacturer OCI recommendations. I've said more than once that most of the people who get twisted around the axle in the oil war arguments don't keep their cars very long. This is NOT a bad thing, unless you're depriving yourself of a chance to retire comfortably and provide for your family, etc....etc. ANOTHER 'you thing.'
