"Well, well, well . . . the 1986 Volvo manual that I ordered off of ebay arrived today, and sure enough what you say about the overdrive selector is absolutely correct. But, man, this thing sure took off when I pressed the [fantasized] "turbo button". I blew past an 18 wheeler like I was driving a Porsche."

When you lockout the overdrive, you force the transmission into 3rd gear. The engine is now running at a higher rpm at which it develops more torque than it does at the same speed in overdrive. Rather like downshifting a manual transmission. I see no harm in using the overdrive lockout in this fashion when climbing a grade or for passing another vehicle. These engines like to rev, and as you know the turbos pack an impressive punch over 3500rpm.