It has been well discussed on the postings dealing with oil, that the Purolator Pure One oil filter seems to provide a good alternative to other filters including the units offered by Honda. I recently replaced a Fram filter that I was using on my 1997 1100 Ace with a Purolator. In looking at the Purolator Pure One filter next to the Fram during the oil change I could see why many Shadow owners switch over to the Pure One "car" oil filter. The Purolators construction and materials looked superior to that of the Fram, or for that matter the K&N I had on before the fram, and the Honda unit before that.
The Purolater was found at the local Western Auto Store for $6.50. The filter, model # PL14610 is sold for cars but fits the Honda perfectly. There are two sizes that seem to fit the Ace, the PL14612 that is the same size as stock unit, and the PL14610 filter that is about 1/2 longer and was the one available locally. I was concerned that the extra length might give me some problems with clearance around the bikes frame, but it turned out to have more then enough room. Not sure if this would be the case with other bike models. Take a look at MarksC posting in the oil sticky for a link on oil filters to see if this works on your Shadow, I was glad I did.