Just to provide my (possibly useless) two cents...I also choked at the $30/qt for Volvo oil and ran a GL5 oil I found at my local parts store for a single season. Every time I would shift into gear when warm, it would hesitate before finally grabbing, which was explained to me was my clutch cones slipping. After reading into what kind of damage/cost that can cause, the $30/qt price isn't so hard to swallow anymore.
I've been running Volvo oil ever since and have yet to ever have the slip return. TL;DR - boating isn't cheap, and sometimes being cheap is the most expensive route you can take...
Just my opinion...