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I know it depends on driving style and conditions but let's assume average conditions. No weather extremes; no bad dust. 75% highway driving some high rpms; some launching, sometimes up over 100 MPH. I know that "they" say synthetic can go 7,500 miles but really? Is 5,000 safe? 6,000 in most cases?
I do not think there is a single answer to this question. Synthetic oils are all different, with different viscosities, different bases, and different levels of the various additives. And engines are different -- operating at different temperatures, etc. And driving styles are different; highway miles are easier on an engine and on the engine oil than stop-and-go urban driving.
Basically speaking, you should follow the manufacturer's recommendation. For my cars it is 10k miles, but for most cars it is less.
I do not think there is a single answer to this question. Synthetic oils are all different, with different viscosities, different bases, and different levels of the various additives. And engines are different -- operating at different temperatures, etc. And driving styles are different; highway miles are easier on an engine and on the engine oil than stop-and-go urban driving.
Basically speaking, you should follow the manufacturer's recommendation. For my cars it is 10k miles, but for most cars it is less.
You're right, there isn't (an answer). Question is ridiculous in the extreme, given number of variables most of which you've mentioned. Same question shows up often on every car forum known to man, with no answer ever provided that remotely makes sense *outside of* professional oil analysis driving the decision.
Professional tuners and builders do Amsoil test kits and similar, to check for wear at intervals. That particulate and viscosity analysis is actually useful to certain professionals, including some of the race teams I used to hang out with twenty years ago (professional motorcycle race teams, with expensive engine builds). I've done 600 mile changes on numerous new vehicles (cars, and motorcycles), then gone to Mobil 1, Amsoil, and other synth brands until further notice. Was eyeballing Amsoil just the other day at the upscale auto parts store: there are a surprising number of varieties for passenger vehicles.
I don't do oil particulate tests, because it doesn't matter for my usage (non-track, non-commercial, non-extreme in any way). I change at certain intervals I'll not go into here, because it's mostly my opinion (vs fact), frankly.
Not sure why that is difficult for people to understand, given one, the obvious variables; and two, that certain parties, notably those crooked oil change shops, have a vested interest in "an" answer that may or may not be entirely scientifically rational.
Professional tuners and builders do Amsoil test kits and similar, to check for wear at intervals. That particulate and viscosity analysis is actually useful to certain professionals, including some of the race teams I used to hang out with twenty years ago (professional motorcycle race teams, with expensive engine builds). .....
I don't do oil particulate tests, because it doesn't matter for my usage (non-track, non-commercial, non-extreme in any way).
I actually did used oil analysis a few times when my primary vehicle was much newer. I had good results with Mobil 1 5w40, then tried a low-viscosity alternative (0w20 or 5w20) that was supposed to meet the required specification but provide better fuel economy. The used-oil test on that low-viscosity oil was terrible, so I immediately switched back, got good results on the next test, and never tested again. The engine now has over 630k miles and runs like a champ.
I run mine for 10K miles, as that's the factory recommendation -- Toyota Prius. I drive 80% highway miles, 15% city, 5% gravel roads (guesstimates), and I drive ~2,500 miles per month, so my driving is pretty easy on the engine.
My guess is, with my high mileage driving and synthetic oil, it wouldn't hurt to drive it twice as far between changes. But I won't, and that's not a recommendation.
I actually did used oil analysis a few times when my primary vehicle was much newer. I had good results with Mobil 1 5w40, then tried a low-viscosity alternative (0w20 or 5w20) that was supposed to meet the required specification but provide better fuel economy. The used-oil test on that low-viscosity oil was terrible, so I immediately switched back, got good results on the next test, and never tested again. The engine now has over 630k miles and runs like a champ.
I don't want to hijack the thread, but what the heck are you driving? And is this 630,000 kilometers or miles?
I also did oil analysis several times on my car for the first several years we owned it as I worked up miles between oil changes. 20k miles was WELL within the realm of safe (so was 25k), we no longer drive 20~25k miles in a year so I simply change once a year.
My newest vehicle has on-board oil analysis for change intervals. It defaults to 10k and adjusts up from there. I'm at 12k right now and it's looking like 15k is when it'll want me to change. If I don't make the mileage in a year, I'll change it annually.
I may not be on this kind of interval if we weren't driving extended highway where the oil can come to temp and cook off the water and neutralize combustion by-products. I doubt I'd ever go under 10k, maybe on a brand new car with warranty...
I don't want to hijack the thread, but what the heck are you driving? And is this 630,000 kilometers or miles?
I've got two 2003 VW Golf TDIs. The one I use for work has over 630k miles and I add nearly 1000 miles each week. It still has lots of original parts, like the engine and fuel injection pump, though I did have to replace the 5th gear in the transmission.
The other one is my weekend car with only about 130k miles, on which I've only added about 12k miles in the last three years. They are both excellent cars and both get a regular diet of fresh Mobil 5w40 synthetic oil every 10k miles -- either Mobil 1 TDT or Mobil Delvac 1 ESP.
Port injected engines can run longer on oil than direct injected ones. My Sonata has GDI and after about 5k miles the oil is black and smells of fuel. I'm kinda OCD so I change it at that point.
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