Oil pastels are for your headsculpts' camo and
maybe some weathering on clothes if you need a greasy/wet effect: oil on a mechanic's coveralls or dark, gooey blood on your zombie, or that nice decomposing look on its hanging innards.
You'll need to seal them, too or you risk smudging your work while messing everything and anything they come in contact with.
But the pastel "chalks" are the thing you want to/can use on EVERYTHING 1/6th for weathering, recoloring, toning-up (highlighting) or complete repaints. Remember you need to seal them or the dust will get smudged all over the surface and neighbouring objects.
Other than that, I don't have many tips to share that would add anything useful to the great tutorials already posted on this site.
Maybe I'll buy a couple of fugly head sculpts and do a step-by-step photo report in the near future, just to prove to myself that I'm not that rusty with the brushes... It's been a while since I indulged complete repaints.