Petrol quality

I know my engine has been a bit on the rich side but my intake valves have an accumulation of tar like substance (on the intake side)
This gummy stuff has caused a lot of problems for many people, have any of you guys experienced any fuel problems? I do buy morrisons petrol so I guess its not the greatest and my fuel does sit in my car for weeks, does it go stale?
The bottom line is the fuel does not have the additives in it that can prevent gumming up, does anyone have any experience of fuel additives?
I was talking to a profesional race engine builder and he said that Shell Optimax does the same thing so cost and octane does not equal quality

ive had a similar problem with two of my classics but being carbs i waz able to clean them out ,but my jag started playing up on the same fuel from sainsburys ,ive switched to esso and the problem haz not returned

I have been reading other forums and it seems that it has been a big problem on Audi, Porsche etc direct injection engines. PCV and valve stem leakage also contributes to valve fouling, took me ages to clean it all off .
Going to put it back together and try and lean the mixture a bit. Any one used redline additives to prevent gumming up?