I am a fan of high quality gasoline, with Techron or a similar detergent. Also, SL/SM rated engine oils with detergents. That is usually all you need.

If you don't use good fuel, or have a sludge problem (a build-up of solids in the oil) then an additive makes sense to clean the engine out. It is always better to keep it clean, but if the engine is dirty then cleaning it can help.

The problem with using additives regularly is that they can contain harsh ingredients that damage seals, O-rings, etc.

If you are going to use additives, detergents are generally easier on the engine internals than solvents. The products noted above are solvent-based, and include ingredients like benzene and naptha (which are usually found in lighter fluid, and Coleman camp fuel, or white gas). Great for stripping paint. Hard on rubber. Yes, they will clean the inside of the engine. They are not discriminatory in what they attack, however, so proceed with caution.

Also, if you are adding one of these products to the oil sump, consider that the base stock of the additive is not up to the standards of the oil that BMW calls for. It will not likely have the correct viscosity, or lubricating qualities. For that reason, one approach is to change the oil, put in the additive, idle the engine, and then drain it and put fresh oil in before driving on it. Putting load on the engine with unapproved oils can do more harm than good.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue