Synchromesh is over-rated anyway. Just learn how to drive without it, and then don't worry. Pretty much all of the semis are that way; just because synchros wear out and they want to go a million miles without an overhaul.
Ok, I'm kidding, but only a little bit. If you learn how to double-clutch, you can't be stranded by worn out synchro rings. And it's actually not impossible to drive without the clutch; I've limped home that way many times (used to own a GM car that liked to snap clutch cables).
Also, from your description and what I have personally seen of the problem; I don't believe you are likely to notice any problem with yellow metal erosion before you are finished moving. It is a very long term problem that takes years and years to develop.
However, there is another problem that can show up quicker : some modern synthetic oils are too slick to allow for good synchro operation. I don't know if the Lucas falls into that category or not.
But I do know that, as the rules are written, almost any oil that meets GL5 is also going to meet GL4! Kind of amazing when you hear the tales of how GL5 is bad and GL4 is good; but the fact is that the GL4 and GL5 specifications are purely performance based, neither places any limits on either base oil or additive package except in performance terms. And the yellow metal corrosion test for GL5 is actually more stringent than that for GL4! It is just more or less coincidence that, in order to meet the performance standards for GL5, manufacturers using that additive have to use a lot more of it to meet the GL5.
Used to be that most GL5 oils were advertised as GL5/GL4; but most manufacturers quit doing that to reduce confusion. Lucas apparently doesn't care about confusion.
Frankly, what I would do in your situation is pick up some Redline MT-90 instead. Order it from Amazon, they'll deliver anywhere in the US in just a few days. It's expensive, but is rated GL4 (only) and is specifically designed to improve performance in synchronized manual transmissions. Also contains none of the additive that can attack yellow metal.
It just makes sense to me to use an oil designed for the application, rather than using a general-purpose "hypoid" oil in a non-hypoid device.
There are other products similar to the MT-90; usually with names like synchromesh fluid, or manual transmission oil. Likely they would work well too; I've only tried the Redline.