As long as you balance your sledge properly, you'll never notice the few extra ounces. I'd suggest an APC 13-6 on the OS70. I had tried a lot of different props on my YS 63 and was never 100% satisfied. One of the Brothers told me to try the 13-6. I'd have to say it made a world of difference in performance. I tried it on a Saito 72 and it worked just as well on it also. I haven't had the opportunity to try it on an OS but I think it would be the prop of choice there as well. The 4 strokes don't seem to respond to the low pitch props like a 2 stroker does.
Matt