Originally Posted By: lovcom
What about oxidation of the oil? Its not like its in a bottle all sealed up, air tight...
Isn't oxidation the reason car makers say to change the oil after X miles or Y months?
For an engine oil to oxidise, it needs to be hot and in the presence of hot, replenished blow-by gas. When your car's at rest, the oil is cold and there's only a static volume of gas sitting above the oil, so oxidation is to all intents & purposes, zero. Even when your oil's at say 100°C and you're tootling along at say 2500 rpm, the rate of oxidation of any modern engine oil is minimal. However if you start really pushing the engine (say you are track driving) then everything gets way hotter and there's a lot more blow-by traversing the crankcase. In those conditions, the rate of oil oxidation will increase exponentially.
The usual OEM recommendation to change your oil every year isn't so much 'fact based' as it is a matter of convenience (once a year is easy to remember!). Also bear in mind that both the OEMs & oil companies are not disinterested parties when it comes recommending OCIs (one wants you to use the best oil to protect 'their' engine whilst the other wants to sell you as much oil as possible).
I liken OEM OCI recommendations to those dentists that 'recommend' you go for six monthly 'check-ups'. He spends ten minutes looking at your teeth, takes a couple of X-rays, tells you're doing a great job of looking after your teeth and sends you on your merry way but not before parting you from £50! The entire arrangement favours the dentist far more than it does you...