Why change synthetic oil?

[Chris Lovell]

I am sure this question is beat to death somewhere on the bbs but I can't search for some reason...

Q: If the main reason to change engine oil is that the oil degenerates over time due to thermal breakdown, especially in our hot running air/oil cooled cars, why would synthetic oils, which do not suffer the same breakdown characteristics as dino, need to be changed? Obviously the oil will darken with micro carbon particles but a good oil filter should take out the damaging stuff and I suspect the micro carbon is actually a good lubricant. I suspect there are some who change the oil at far longer intervals than suggested but replace the filters more often.

And lastly, has anyone reused their Mobil 1 or other synthetic in other engines like a lawn tractor or whatever?

Opinions welcome. Thanks.

[IROC]

Synthetic oil still breaks down just like dino oil - it just lasts longer. Plus, it gets dirty just like dino oil and needs to be changed.

I'm not exactly sure (it's been a long time since I've looked into this) but I think alot of the sythetics on the market actually use the same "base" oil as the dino oils.

Mike

[Emission]

BMW changes the factory-fill synthetic oil in their cars (for free under their maintenance program) every 15,000 miles or so. Synthetic is good for extended intervals, but it does get dirty and need to be changed eventually (like IROC says).

I use synthetic in my 930 because it can handle the heat of turbo bearings better than mineral oil (dino). I change mine in my Porsche every year, regardless of miles (it works out to about 4000 - 5000 miles between changes).

I run synthetic in my Infiniti and change it every 7500 miles (every six months). I think that is a bit longer than the recommended 5000 mile interval by Nissan. I'd rather pay another $20 for the synthetic and stretch the interval because changing the oil is a PIA (all these panels need to be removed under the front of the car).

[dickster]

I suspect there are some who change the oil at far longer intervals than suggested but replace the filters more often.

no, for me its the other way round.

[ChrisBennet]

One of the by products of combustion is acid (sulphuric?). I think the additives in the oil neutralize the acid until they are "used up".
-Chris

[deoxford]

Audi changes the factory-fill synthetic oil in their cars (for free under their maintenance program) every 10,000 miles

Everyone here is correct, I have read that some synthetic can last as long as 20,000 miles But I think every case is different.

I work for Ashland so I going to do a little research to see what studies have been done on the subject.

I will post the info when I get it

[Richard C2S]

I use Mobil 1 in four cars (one is a turbo) and change the oil much more frequently than the manufacturers call for. I use the Mobil1 because it will (or, is supposed to) hold up longer and provide better protection from heat related break-down. I change it every 3 to 4k miles for peace of mind. It's a lot less expensive to change the oil frequently than to replace turbos or rebuild engines. Just my $.02, which is all I have left after buying all that expensive oil!

[Bill Verburg]

As chris stated by products of the combustion process get inot the oil and condensation. The more short trips, the more of both.