I've found NP1 very nice for sealing the mortar cuts for counterflashings on chimneys. I refuse to use any type of mastic in valleys. Never have had a reason to. I guess if I were going to, I'd definitely recommend an sbs modified. (Henry's 908 or the like)
Just bend a return into the valley sheet. No need to seal them together. A sealant can actually cause water to be pushed into the roofing system at valley locations.
__________________ www.roseroofing.net/ Free inspection within 12 miles
Seamless Polyurea Roofing Membranes for Metal Roofing, Roof Leak Repair, Seamless Commercial Spray Foam Insulation, Seamless Commercial Roofing, SPF Roofing, Industrial Insulation, Our trucks are red but our roofing is GREEN!