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.)
