90 740 base w/133K miles here.

I just switched to synthetic, mobil 1 tri-synthetic 5w-30. Also got a better filter. I've noticed that the car runs quieter now, especially during idle. That could also be related to my engine mounts... who really knows. I'm pretty sure it was due to the synthetic change though.

It's definitely worth it. But before you switch over, you should check to see if you have any oil leaks anywhere. Synthetic has the properties of both cleaning out gunk in your engine, as well as seeping into leaks that you never knew you had. This can result in leaks. And if you have a tiny leak, it might get bigger, because the synthetic might clean out the gunk that was sealing up a leak. So due a quick check. ***edited***

Besides that, I would definitely switch. I sleep better at night knowing synthetic is in my engine now. :-)

As for gas mileage increase, I'm not sure about that yet. I know it's not any worse off, that's for sure. I'm still on my first change to synthetic, so it's still kind of "washing out" my engine at this point. I plan on doing another change in about 1500 miles, and then 3000, and then maybe 5000. We'll see how things go. :-)