Hi All,
Many newer mainsprings come pre-lubricated with a teflon (I think it’s teflon or something like it) coating which they say doesn’t require any lubrication. That said, opinions do vary about the need to add a couple of drops of oil or dab of grease to the top of the spring after installing and before putting the cap in place or just to leave it as is. I’ll just add here that there’s always the danger of oil running out of the barrel and wreaking havoc in the watch so… 
Bob