GAS ADDITIVE/SAVER
 

Have you ever used one of those???
I also saw one in BMW accessory catalog but I'm not sure whether it does help alot... :rolleyes:
(http://accessories.bmwusa.com/ItemVi...&productId=692)
Any advice or recommendation?

Generally, if you use good quality fuel (with sufficient additives/cleaners) you don't need it. If you have buildup of deposits in your injectors, it can help restore smooth running.

Chemically, it is reported to be Techron, also available from Chevron retailers.

I believe it is just Techron additive repackaged as BMW:

Our Fuels: Fuel Additives: Techron Concentrate Plus

The use of these additives is a never ending debate, sorta like what is the best engine oil.

I personally use a bottle of the stuff every 10,000 miles or so.

While there is little consensus on the use of additives, if there is any agreement at all, it is that Chevron Techron is the preferred additive, if you are going to use one.

Then...
would there be any possible negative effect of using additive?

You don't want to run too much of it. I would use it and run the tank of fuel before an oil change. It can dilute the oil if you use too much of it...

Craig