I use Shell Rotella or Mobil Delvac 15w40 in my 5.7L Indmar with a Purolator PureOne filter. Both are cheap and have a really good reputation. I use the same oil in my Hurth V-Drive/Transmission too (Many take ATF or have separate reservoirs for transmission and V-drive lube though).
If your manual specs 15w40 there aren't that many easily accessible options in my area. There's SuperTech 15w40 at WalMart and Royal Purple 15w40 at AutoZone, NAPA, etc. Rotella and Delvac are available just about anywhere that sells oil.