Vaseline and greases are insulators, but that doesn't mean they won't benefit a connection. When you clamp up a lug or screw a conduit joint together it might look like there are a few square inches of metal-to-metal contact. Instead, at a molecular level, the total amount of surface in contact is more like the head of a pin and yet that is where all the current flow takes place. The grease's job is to exclude moisture and air so that those tiny conductive paths don't oxidise and corrode over time.
The mechanical pressure on the contact points is huge so any grease is well and truly squeezed out of the way into the voids that form most of the visible surface, so it make little difference whether it is an insulator or conductor itself. Grease with suspended metal could form additional paths, but copper particles on a steel-steel joint could also encourage the kind of electrolytic corrosion that the grease is there to prevent.
Aluminium is a different kettle of fish and not to be compared with joints in copper or steel. Exposed to air, it forms a tough insulating oxide coating almost instantly, so it needs special techniques to make a reliable connection. That is one of the reasons why it was unsuccessful for housewiring.
If your new conduit joints are having an effect on your Zs, you need to quit doing them up with a clothes peg and buy a wrench! It's after a few decades of exposure that the effect of the grease begins to show up, as a corroded threaded joint can go completely open circuit.