Try a search and you will get a ton of results on this topic. I think issue #29 also had a write up on additives.
To answer your question:
Power Service (the kind at Walmart) is the number one additive based on sales numbers alone. It is a good additive, IMO, but be careful as they make a few different additives and I don't remember what color bottle is the best. It may be white.
On this forum, Stanadyne seems to be the most widely used. It is as excellent additive IMO, and it is approved my many major engine MFG's, but not Cummins. Prolly casue they make their offer their own, which is OK I guess but overpriced IMO.
LE DCI+ is also a top notch additive. I have gone through many bottles of it, but it isn't as availble as others.
Howes Meaner Power Cleaner has been talked up lately and I am trying to locate some locally to give it a try myself.
You are on the right track. An additive should always be used, IMO, regardless of the time of year.
-Ryan