Gearbox oil additive yay or nay ?

I'm about to drain my KZJ90 gearbox and have been looking at treatments and additives , seem's there are two schools of thought on this for and against . But hey a Lancruiser is a different beast altogether so may i ask your views .

http://www.halfords.com/motoring-travel/engine-oils-fluids/gearbox-oil/molyslip-gearbox-protector

I've never used them, but if you're having some gearbox issues I don't see the harm. However if your box is fine, is there any need?

No issues as such but found selecting reverse a pita yesterday , its never been good and i often have to let the clutch go a little before it will engage . The stick just feels a little too mechanical lately like its not wet enough . I plan to change the oil 3 times this week to dilute the crap that can't be drained and just thought with 100k+ miles on the box there has to be some wear so maybe a little extra like the Molyslip above wouldn't go amiss .

I just have doubts about the pro's and cons - ignoring the hype how do these additives work ? if it changes the viscosity of oil then surely its the same as using the wrong oil ?

I'm not too sure on how they work either. Maybe they change the adhesion properties of the oil so it holds onto the internals and gears more than normal? Better thermal changeover properties?

Considering these boxes are pretty ancient, I think good synthetic fluid should do the trick! Something formulated for higher temps than "normal" would be a plus. Flushing it as you've suggested will do a good job at cleaning the internals out.

With regards to your selecting reverse, have you tried to see if there any adjustments? I know shifters normally have rubber boots and sometimes these can wear and make it harder to find the gates?

No problem finding reverse it just doesn't want to mesh ?

The oil was perhaps a little darker than it once was but not as dark as my eye will be if she sees her kitchen sieve in the shed happily there was nothing left in the sieve and reverse seems ok . Still wondering about whether or not to use additive's though .

Is yours a manual? Using redline mt90 made a difference to mine, and took the edge off a whine it had.