Luckily I have a friend who builds and races Rally cars with his own workshop within spitting distance of where I work, so once my car's run out of warranty, money can be saved there paying "mates rates"
Here in the states that's a six pack and a pizza. What's the rate on your side of the pond?
My '11 Subaru specifies an OCI of 7500 miles. I will adhere to that until my warranty period isover and then will immediately go to 10,000 miles. I may extend that to 12,000 but at 10,000 I will get an analysis of my oil. If it comes back good I'll start using 12K miles as the new OCI.
I do all of my maintainence work on my cars because I don't really trust others to do the work correctly. A DIY oil change costs me approximately $30 (and about 30 minutes of my time) using top-of-the-line synthetic and OEM filter.
I don't really know why some people still insist on 3K OCIs. Back in the 70's engines only lasted 100K miles before needing a rebuild and OCIs really were 3K miles. Nowadays, completely unnecessary and massive overkill unless you live at the end of a 20 mile dirt road.
IDK, it ran well, but the valve guides on the #8 cylinder were worn. Every oil change I used to pull the #8 plug and spray it out with some carb cleaner and recheck the gap. Never had any visible oil burning from the tailpipe though. I would not have felt comfortable extending the oil change interval considering the oil and filter technologies prevalent at the time. I still go by the 5,000 mile interval, but I use better filters and synthetic oil now. My 98 GMC pickup runs so smooth and quiet that with almost 200,000 miles on it you can't hear it idling from 10 feet away.
I'm still sceptical whether it makes a difference. I mean sure, any car would run smooth for decades if you changed the oil every few months, but unless you've done hundreds of thousands of miles and changed the oil every 20,000 or 30,000, how do you really know if it makes a difference.
It tends to be smaller engines need more frequent oil changes too, so it seems even more excessive that US drivers use close intervals when the engines tend to be 3 to 5 times larger. As mentioned before, a large haulage truck in Europe, big engine lots of miles, has oil change intervals at 60,000 miles. I might ask my friend who's a courier driver how often he changed the oil on his van. He drives an 06 Merc diesel van, he must be close to 700,000 if not 800,000 miles now. Admitadley he has rebuilt the engine once, but I think thats expected with this extreme mileage.
Engine displacement has very little to do with oil change interval requirements. There are many, many variables that deplete the additive package in new oil to the point where acid formation will cause damage from internal corrosion.
Google "oil TBN number". Total Base Number is the quantification of the oil's ability to neutralize acid formation during engine operation. Contrary to popular myth, oil DOES "wear out" as well as get dirty. The goal is to change the oil before the TBN is less than "1", and thus has worn-out.
And another interesting Google research project is "Synthetic Motor Oil defined". "Synthetic" is more a marketing term, than an indication of characteristics of the oil itself.
The basic lubricating qualities of the oil itself don't wear out. It thicken over time from various causes, but will still lube.
The additives do get used up. The acid neutralizers that make up the TBN are the ones of usual concern. Contaminates from the air or fuel, along with scorched oil particles from hot spots in the engine, cause the oil to get 'dirty'. These particles are generally benign on their own. Large ones get caught in the filter, and detergents in the oil keep the smaller stuff suspended and forming sludge.
However, these particles can form acids when mixed with water, and water will always find its way into the oil. When this happens, the acid neutralizers do their job and get used up in the process. When used up, the acids can start attacking materials in the engine. Even with the TBN just low, the acids can cause some harm before being neutralized.
There is also antiwear additives that get used up over time. They prevent damage at times when the available oil itself isn't enough. Start up is where they obviously get used, but it also happens at other times. GM's oil monitor system doesn't look at the oil in the car at all. It is making a calculation on oil life left based on the known consumption rate of a common antiwear additive at various air temps, engine temps, and rpms. It has shown to be accurate on that calculation based on some used oil analysis.
Oil change intervals in the US are shorter simply due to marketing and tradition. IMHO. Our gasoline may contain more sulfur than in Europe at present, but seems a to have minor effect. People using oil analysis to determine when to change their oil have gone over 10k miles between changes. Depending on car usage and climate, GM's OLM may even say the same.
There are 5 classified groups of base oil. The first 3 come from refined petroleum. Group IV are the synthetic PAOs, and group V is a catch all for everything else. Synthetic became a marketing term, at least in the US, when Castrol successfully defended its use for a group III containing oil in court on the claim it provided the benefits of a full group IV one. A synthetic labelled oil in the US should be better in terms of properties than a non-synthetic one. 0w ones will have some group IV or V base mixed in at least.