What is preferred/best contact cleaner these days?

Lol

yeah I thoroughly read every word of the op. May be not…

I've been using 99.9% Isopropyl Alcohol.

Then a little High Fidelity Cables NPS-Q45T Nano Contact Enhancer.

for anyone following, make sure that you use a contact cleaner that is ALSO a LUBRICANT.

crc brand of deoxit

And, there are versions for faders, plastics

deoxit fader

WD40 is NOT a lubricant, it dissolves rust/corrosion, that's it.

Audio Classics does not clean sliders with felt or foam strips, the chemicals dissolve them, they suggest just working them full range frequently.

OR, skill/tools needed (if/when a diagnosed problem) take them apart, plan on removing and replacing felt or foam.

Around the edge of vintage back-lit tuner faceplates, the foam is often dissolved by age, and disturbed by disassembly. I used a black shoelace to block edge light for my Fisher 500C’s tuner glass. Another one, I improvised, some soft u shaped strip that pushed onto the edge all around, something hanging around the shop..

Be very careful cleaning the tuner glass with printed info, very light touch, a camera-lens cloth, perhaps a speck of Windex on that smooth cloth, no other chemicals or textures, avoid perfection here.

Check out Stabilant 22A. It works by filling in the micro-gaps between the conducting surfaces on a molecular level and then sealing the contact from the atmosphere. It is expensive but very effective. Used by NASA.

(I read about it on Audiogon two years ago and have no affiliation.)

Micro-gap filling ? Talk about diminishing returns, there’s one for the ‘tweaked’ audiophile ears. Most can’t realize a difference doing an A/B cable test. Pastes, pads, contact crap, magic rocks / beans, it all makes a difference ? Laughing is good, no BS there.
Cheers