I remember when a lot of racers use dish soap to coat the insides of their slicks, I have done that but forgot about until seeing this post
It worked but Hoosiers tubeless slicks do hold air a lot better than M/T tube types slicks due
My last set of 14/32 M/T stiff walls slicks held air fairly well when new, once they were worn to less than 1/2 the original tread depth they started loosing air quicker
I ended up buying a new set of tubes for them and ran them until all the wear holes where gone