Maybe try an AutoRx flush for your engine...
http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=5;t=003022Quoted from BITOG...
...Then a high quality oil. Ex: Castrol Syntec "Made In Germany", a very unique oil in its use of PAO base and esters with cutting edge technology.
http://home.comcast.net/~dsmphotos/gc/gc.htmlI used to use Mobil 1 but now am a convert to Castrol Syntec. As with M1 use, I check my valve covers, they are clean. Quick high-spin starts in -25 C weather, good oil temperature, coolant temperature and my engines are quiet when they idle. What I like about this GC is the oil is towards the high end of the 30 weight scale, so when I run my car or truck hard it handles higher temperatures. Good for seals.
I would try to be more educated about "additives". Few offer return on investment, many deliver what they say they can but at a cost of reliability elsewhere.
Not too much as good as your choice of a quality oil. Pretty well any oil that is SL or the newly SM rating....does not require additives even for older engines. For older higher mileage engines a good conventional oil will do just fine when performing regular oil change intervals.