Synthetic oil in a rarely-used engine -- lasts how long?

2003 Subaru, got about 23,000 miles on it (a few camping trips each summer, otherwise we rarely drive). Did the 30K service a few years back, based on years – had synthetic oil and auto. trans. fluid put in (by a shop mostly working on race Subarus; we do climb steep forest roads with camping gear).

It’s been about three years and about 2500 miles; how long can synthetic oils last in light use? Does the magic evaporate even when the car’s not driven much?

In a sealed crankcase, motor oil, both mineral and synthetic will last a LONG time…Chemically, it’s very stable. I wouldn’t push it past 5000 miles, that’s long enough for ANY oil…

No, the magic doesn’t evaporate, but neither does the water that gets into the oil as a byproduct of combustion. If the oil is not changed often enough, the water turns some of the oil to sludge.

If you don’t drive it often, you should at least change the oil once a year, synthetic or not. And in that case, using synthetic oil is rather pointless. Use regular oil and don’t wait 3 years again to change it.

I think it was on it was written that additives lose their effectiveness over time, even sitting in the bottle in your garage.

I had never heard that before, but if it is true, sounds like three years is long enough. it won’t break you. I do not know if that is correct, though.

If you want to see the effects of moisture on oil put some clean oil into a shallow pan and add a little water. Allow it to sit outside (under cover) for a while.