The manual calls for 10W.
To firm things up a little, I used 80% 10W and 20% 20W, with Progressive Springs. Worked about right for me, but I'm 190lb.
You can use 15W, but you may find that the forks become less responsive, or harsher with too much compression damping. Since these damper rod forks are pretty simple, too heavy an oil just make them harsh. In this case a little may be good, but I think 100% of 15W would be too much. However, depending how hot it is when you ride, you might find it OK.
As for spacer length, Progressive suggests even with the top of the fork leg, with caps off. I found this a little too light, and am adding another 1/2-3/4" this spring. Easy to cut several different lengths using poly tubing. If your going to cut down the stock metal spacers, you can always use a large washer (or 2, 3, 4) to fine tune.