I use synthetic, but I don't use the clutch to shift. Synchronizers need friction to work and from what I hear, synthetic is too slick to work. If you depend on synchronizers, 90W mineral oil would probably work better. Stay away from GL 5 because it will attack your brass synchronizers.

I'm not sure what you mean by lurching, but I would think the clutch is suddenly grabbing and the synchronizers have nothing to do with this. Either you have messed up clutch linkage or a rough main drive gear (input shaft).

I would take the flywheel cover off and get under it and watch with a flashight to see what your throwout bearing does when someone steps on and releases the clutch. Make sure the motor is turned off and the wheels blocked when you do this.