Gloss or flat for streaking?

I use a flat finish.

When you say streaking I assume you mean DOT filters where the oil paint dots are dragged down to simulate rain marks and weathering. I use a flat base so that the oils blend better than on a glossy base.

Depends on whether you want streaks to look wet or dry.  Glossy streaks look wet, good for oil leaks, water leaks, etc.  For a water leak I sometimes just use gloss clear.  Most rust colors I use are flats, so if I want the rust streak to look wet, as for a spot that often has water coming down, I put the gloss clear over the rust, but not in perfect register.