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additive has been used since the 1950's

The additive was used to modify the slip factor of GL4 lubricant, increase its slip factor, and stop the limited slip clutch plates from chattering going around corners. Whale oil was used in the additive. The use of whale oil stopped with the whaling ban.  The new stuff has never worked as good.   my opinion

The clutch free play of 1 inch is used to allow for wear of the clutch components in particular the disk. As wear occurs free play diminishes.  Setting the free play near zero and driving the car [normal driving] will allow you to see if setting of free play or amount of pedal travel is your problem. If pilot bushing or clutch itself is the problem the minimal free play usually does not help the problem. Do not try hard acceleration with minimal free play as clutch damage can occur.

It is true there is no syncro in reverse that is why you use reverse to diagnose a clutch release problem. Depress the clutch, engine at idle, wait 3 to 5 seconds depending on engine RPM, shift directly into reverse and if gears grind you have a clutch release problem!  The problems you describe are typical complaints for a clutch problem. Remember the pilot bushing, an old worn or a new but dry bushing can cause the transmission input shaft to turn with the engine causing problems you describe.   Yes I have a 67 coupe I have been working on for a very long time

Sounds like you need to replace the clutch and pilot bushing. Try shifting into reverse while stopped, if transmission grinds after a short wait time clutch or pilot bushing dragging is your problem. If no grinding and reverse goes in smoothly maybe the  new lube is your problem.