Whats best when applying top coat oil based satin wood... good quality brush or gloss roller. In the past, even with brushing out well, had problems with runs in panel corners and in bottom of panelled radiators.
I always preffer a brush, Far better finish, Just dont throw the paint on keep the coast even and put a little drop on at a time, Allow about 5 min for runns to occur and then just give them a dab and brush out with a dry brush. Everyone gets runs at somepoint sometims it just fills down the edges hens why it builds up in the corners.
Radiators i always use a brush, as a roller will not get in the rite place, as for doors i use abit of both, for the flat panels i would use a gloss roller as its quicker and alot easier to get an even amount of paint, if the doors have a wood grain effect i would first roll them then run a brush over them after to keep the grain.
As for runs, just keep checking it every few mins and brush away whats there, if your getting alot of runs this is a good sign your using way to much paint.