Just too cover the basics...
i think most RB's take ~4.5
I have used Ryco filters no issue just because its made in China it doesn't make it shit, i would say most car parts are made there, just different factorys turn out different quality products, on a rb30, rb25 it is Z145(i think) not sure about RB20's tho...
If you're filter isn't getting tight by hand, a scrap of sandpaper around it to grip it will assist in getting it tight enough, Also, Rb25's are AW12 sump plug washers... could be worth a new one.