Check the gap between the two outer shells. The one on the steering wheel and the one on the column, should have at least a 16th inch clearance. Perhaps more for the Grant, if there is any little bit of “Wobble“ to the housing.
Does it make the sound 360°? Or just partially?
I’m a fan of grease. I always use it even on the factory wheels.
The horn contact contact can wear quickly without it, but so can the far too thin copper traces on the back of the wheel.
I think I used to use dielectric, but with your setup with the single trace and the horn relay, you could probably use any long-term long life grease. Maybe silicone based grease?
Hopefully somebody will know the proper product to use.