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Assuming you’re addressing me, I agree.. sorta. 
I often add lube in a liberal fashion.
It’s not with ignorance that it may attract hairs and dust, simply that I sometimes believe things may not much life left in them anyway and so need to work for then or as long as they can.
Things that cause friction without using lube get damaged more quickly.
It’s just a fact.
Those who panic about where lube is in a slider, probably aren’t using the best lube to begin with in my opinion tho.
I’ve seen lots of pots displayed on scopes using DC in and applying lubes and just not viewed the horror stories.
Quite the opposite really- most have quieted up and felt better.
Lubrication isn’t the only thing to perform besides cleaning anyway.
Lower friction can be achieved by burnishing or polishing the wipers and contact strip before adding any lube in the first place.
Well polished, tensioned and aligned contacts are best in my opinion.
Then how much lube is present is moot. (You ARE keeping your gear covered, right?) 
I’ve not found any lube that has noticeable dielectric effects.
Perhaps they’re using some sort of conductive grease or their pots aren’t wired correctly.
(an input from a potentiometer not wired as ground/wiper/signal in a high gain circuit, for example.)
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#2, my Opus has some bleed through on the Organ sound, meaning that I can have the right slider all the way towards Brass and still I hear the organ playing through even with all of the footage sliders down. Do you think this is likely a slider issue (or multiple slider issues) or something else (I’ve seen the long list of upgrades you made to the caps and ICs on your Opus restore). I’m no where near being able to diagnose issues but am perfectly capable of replacing components.
Thanks in advance.
I can’t say. Your amp, grounding and other issues are unknown to me.
If your synth is all apart and normally uses a steel panel, extra ground wires and other things normally in place, don’t be to surprised it might act odd tho.
You could verify igf the grounding node at the volume sliders is grounded by shorting it to the main power supply or audio out grounds to see if it gets quieter. Various small experiments like that.
But look at everything and see what you’re up against.
A slider might be ungrounded or have poor end of travel resistance.
Wires might not be shielded in places that normally are or one bad ground pin somewhere could cause such a problem.
Perhaps even put it all back together where extra grounding points are made, jiggling connectors, using different amps or headphones, etc.
Fwiw, I don’t suspect parts first. Especially non-electrolytic caps with no visible damage.
I suspect physical contact first.
Good luck.