Pretty sure it's all mileage based. There's no oil quality sensor that I know of, definitely not on a B5 and I highly doubt it on a B6. My B8 is also mileage based, it does however have an oil level sensor that displays on the MMI screen, there is no dipstick.
I change the oil in the B5 every 5000, in the B8 I'll just go with what the car tells me, aka manufacturer recommended interval.