I have a 1998 BMW 528 and I used synthetic oil once. Can I switch back to regular oil?
You can switch back and forth as much as you want. Just make sure that you use oils that meet BMW’s spec for your car.
You can switch back and forth and even blend them if you’d like.
Synthetic oil and regular oil are chemically exactly the same. The differences are that synthetic has more consistant molecule sizes (allegedly, I’ve never actually seen them) and fewer impurities.
Are you sure your car allows conventional oil? I know that the 5-series a few years newer than yours requires synthetic oil.