isnt the Wynns step a bit of a mine field?
Im not an expert at all but I've been reading all sorts and the general consensus has been, if you have a new car or its had plenty and regular oil changes then its fine, but if its a bit long in the tooth with unknown oil changes then doing it may give you more hassle as it breaks up all the old crap, which may either lead to leaks or more likely block things up as it comes off and floats about. I read somewhere that if your going to do it on poorly looked after engines or long oil change intervals, then the first time you'll need to clean it change it, run it and repeat it 2 or 3 times to help reduce the chance of crap getting in to places you don't want it, suggestion was to use cheap oil first for a couple of hundred miles then on the final go put in the good stuff. Using it after should be fine so they say...
We have to be clear that we are talking about fuel or oil additives, you sound like you are talking about oil additives Lenny is talking about fuel additives (obviously 2 different things)
Fuel additives such as Winnes cannot clean the EGR valve, or inlet manifold, they can only clean the fuel lines and injectors, combustion chamber etc, the fuel goes nowhere near the EGR valve or inlet manifold on a direct-injection diesel, to clean the inlet manifold (or EGR valve) you need to take it off and clean it properly