Granted, there is an awful lot of advertising hype out there that the oil additive companies should get sued for and I'm certainly not defending them. However, for what its worth, when I was driving my 240D on a daily 80 mile mostly freeway commute, I consistently got 24 mpg and if I added STP to the 15-40 oil at a 3500 interval change, I got 26 mpg. I ran this test a number of times over a couple of years so I know that it was real. That increase certainly isn't going to make anybody rich, but it surely will pay for the can of STP and a decent lunch.
I can't tell if it affects my E car as that one is subject to varied driving routes so I can't do a controlled test.
Frankly, modern oil is so good that anything that is added to it can at best only produce a marginally better product, but some things can easily produce negative effects by having incompatible ingredients working against each other. We simply don't know what ingredients an oil manufacturer has used and we also don't know exactly what is really in an additive, so we are guessing about the safety to our engines. I think I'd get a lot of agreement that the best thing for an engine is to change the oil on an advanced schedule.
'85 300 SD, 165K-Sold
'88 300 TE, 165K-Sold
'64 Porsche 356C Cabriolet
'86 560SL 124K Miles
'94 320E Wagon, 74K Miles