This lock I'm currently working on has an issue with the frizzen not always opening. I polished the spring to 1500 (No...I did not start at 400 and jump to 1500....who does that?). I've polished the frizzen toe the same. I've studied both the spring and frizzen toe for contact and shape, but this lock will gouge (not scrape, not mark, not mar, GOUGE) the spring when left to just a quality polish. YOU can polish to an 80grit if that's what you like and it works for you. I'm going in steps to 1500 FOR THIS LOCK because of the fits it's giving me in gouging the spring.
Why? I can't tell you why. My 3 other flintlocks need no lube on the frizzen spring. Not a drop since 1979.
In addition, it doesn't matter what they did in 1785. The metals poured today are different than those in 1785. The metal they poured in 2020 is likely different than the metal poured in 2024. The metal in the lock I'm working on is what it is. Oil makes it work. Grease makes it work as well. Grease is MY choice to use. You or anyone else can use whatever you like. I will use grease.
I believe the reason this lock does the gouge thing is the spring metal is not the same quality as my other three. That's the only reason in my amateur opinion why this occurs. Once this lock is installed in the pistol it's going into, time will tell if I need to replace the frizzen spring or not.
All my input is geared for is to try and help the OP fix this maddening issue. This is what I've done and it works.
But you do you. That doesn't mean what I'm suggesting is wrong.