More a word of caution than a problem.
I bought my car back in Feb, one of the first things I did was an oil change (put in GTX Magnatec), at the same time I used the product called "Greased Lightening" it comes in 2 bottles one is a fuel treatment and the others a PTFE (teflon) additive to put in you're oil to reduce friction.
Anyway did this and have been happily driving the car, I did notice that the engine was a little rough and sounded quite noisy, but assumed it was because it had done alot of mileage (92K) no real problems as such.
Yesterday I decided to do an oil change again (only done about 2K miles, but I was bored). Replaced it with some fresh GTX Magnatec, took it for a drive and I was stunned! The car felt like it was floating, honestly I couldn't believe the difference, the engine is smooth as a babies bottom, quiet as hell and generally brilliant to drive. The thing to note though is because I had added this stuff to my oil as one of the first things I did, I didn't realise that it had made things worse since I didn't have a baseline to compare to.
Right I'm boring you now so I'll hurry up, I can only conclude that the PTFE additive didn't like the Magnatec oil and reacted with it (I did notice the oil had gone green coloured but didn't realise) thereby reducing its effectiveness.
So if you are tempted to add one of these additives to your engine oil bear this in mind.