life, easy highway and steady low RPMs extend it). Pending I have a quality synthetic oil and filter, I trust it until it gets to 10% or even 0%. That might be 6 or 7k if I'm towing heavily, or I've seen as high as 12k when crusing.

My BMWs are the same way.

My Toyotas just go by mileage. Whatever. I never trust those, my synthetic blends I change around 6-7500 and my full synthetics I change at 8-11k. Again, depending on use.

Going by "time" doesn't make sense to me. I've had oil in vehicles for well over a year, it doesn't go bad or expire.

The problem you get is there is a chance moisture can condense in the oil if a vehicle sits, and that can be trouble (for large electric motors at my job that we "lay up" in storage, we always do an oil change prior to placing them in service primarily to combat condensation).

For a car though, if you drive it occasionally at temp (no short cycling) so the moisture evaporates, it won't be an issue. Keep the quality synthetic in the car for 7-12k miles, whether it takes 24 months or 2.

If you really want to be paranoid, send in samples and get it checked and if your filter is doing it's job, you might be able to get 14-15k out of quality synthetic.