It's often mystified me as well - but I think the reason is, in dino oil GL-5 will work ok, won't eat the brass all that fast (if at all really), and is easier to stock at the dealers'. Stocking multi-grade oils is a high cost proposition. Don't we already complain about dealer costs passed on to the consumer?

The problem tends to emerge with synthetics. Watch and see if nearly every complaint occurs after someone goes synth, after running successfully for years on dino GL-5. Synths are more 'slippery'; the shear strength and lubricity prevent the synchros from grabbing the gear cones. Even some synthetic GL-4 oils are too slippery for the synchros, ask me how I know!

If going synth, by all means pay close attention to the grade: use GL-4 exclusively. Redline manual transmission gear lubes have proven highly successful in getting older trannies to shift like new. Better than new really, freshly rebuilt trannies often don't shift all that great until the synchros break in.