Yes and donno, I use silicon spray 

1577113912_6444324525e00d938068f36.75290203_blaster-car-fluids-chemicals-16-sl-64_1000.jpg

 
Well there you have it, 100% agreement 😆

So many variables, you just have to experiment to see what works best.

Assuming we are talking about a PCP, you can try most any common lubricant. With a springer, avoid anything that can ignite. The instantaneous temperature rise will cause dieseling and can damage the piston seal and spring. Not that slugs make much sense for springers, only mentioning it just in case.
 
Ok, I did try a short answer jaja

it will depend mainly on the barrel and the slug size and weight. Airgun barrels are not standard in bore dimension, and results might be different too.

pellet have like donno 30% of the surface touching the barrel, slugs is more like 80% (not exact percentages just to have an idea) ergo, more friction, ergo lower speeds. Lub on slugs help to reduce that. But also will depend on the slug diameter, barrel choke, etc. Lub It might or might not influence the accuracy (in a good or bad way) that is a totally different story.

It is needed? I believe the only one can answer that is you. Try, test and let us know, like you can see here there is not a definitive answer it will depend.