78 Coupe d'Volvo. It used to work. Now it doesn't. It failed slowly, requiring higher and higher speeds in which to engage it. Even with the clutch disengaged, which I've always done because it's muscle memory and I accepted that. It continues to slip. When hot particularly. I'm about to flush it, replace the fluid, do it again, and if that doesn't work I'll replace the fluid with something heavier like engine oil. Should I not do that, and what else can I do.

 

that sounds like a worn out lycoming J unit (the actual overdrive). at this point, it probably needs a rebuild.