for many years I used something between 1,5 and 2, once per week. Sometimes I used derma roller and other times I used derma pen.
Recently, according to some researches (but they are ambiguous so take my conclusion "cum grano salis", I'm using 1 mm, every five days.
I see an overall increase in density/covering effect. I can't say it is because I have more hair or ticker hair or both.
On the temporal peaks, I see an increase in baby hair (it can't be different, because the TP are made of baby hair).
On the HL I see new hair coming out.
At the beginning, about 5 years ago, I used a mixture of olive oil and castor oil.
Since a couple of years, I'm using minoxidil.