Well, first off, I'm with Nick. Use whatever makes you happy and is pleasing to
your eye. Acrylics, oils, printer's inks, hobby enamels - they're all good.
Eric, when you say "I think that silver or gold metals could be paint with metallics and silver or god laces or fabrics could be paint with oil", are the 'metallics' metallic oil, enamel, or acrylic paint? I'm just not clear here...
I started off using Humbrol, Pactra, and Testors enamels on what few figures I painted back in the 70s. After a long hiatus, I picked it up again in the late 90s - maybe about 2000 - using Humbrol, but switched to Andrea acrylics shortly thereafter (another story). I found the Andrea paints acceptable for most applications except their metallics. They were thick, lumpy and didn't seem to brush very well. Thinning them just seemed to make the pigments settle out faster. They also tended to settle rapidly in the bottle where the pigments would fuse into a nearly unmovable mass. I discovered Vallejo acrylics were better and augmented my Andrea stash with them. I picked up on Reaper acrylics about 5-6 years ago and haven't looked back. I use the metallic acrylics for chain mail and yellow metals without issue. For white metal plate armor and weapons' blades, I buff the metal with a motor-tool, a napped buffing drum, and jeweler's rouge. The polished metal is then stained with black-brown oil paint, which is allowed to sit for about 15 minutes before being wiped off. Areas that catch more light are buffed more and have more of the oil paint wiped off. There have been exceptions, but I generally stick with these techniques for rounds and flats from 54mm and up. That's just me...
Over the years, I've seen no shortage of beautifully painted metals (along with an assortment of clunkers) using both metallic paints and/or non-metallic metal (NMM) techniques. I don't
generally attribute this to the medium, but rather the painter's skill in
using the medium at that time. As we all know, there is no substitute for picking up a brush and practicing. Where the medium itself might into play depends on the quality of paint and the size of the metallic pigments in the paint. Large-grained pigments give the metal a grainy look (another issue with Andrea*). The smaller the pigment, the less grainy the end result (one reason why printer's inks were so popular). Pigment size is magnified on smaller scales - paint that looks fine on 70mm pieces may not look so good on 30mm pieces.
As for NMM techniques - using an assortment of grays, blues, and white to render white metals and browns, yellows, and white to render yell metals, my mental jury is still out. For the most part, when properly done (practice, remember), it looks fine, my eye sees metal, but my brain sees 'paint'. It's not metal, but a rendering of metal, much like you would see if an artist was rendering an illustration of sword blade - or a chrome car bumper. To
my eye, well-executed NMM works best on 30mm pieces.
I suppose I could blab on some more, but it's after 5 PM and dinner is calling. I am never late for dinner...
Peace out, yo
Glen
*It should be noted that after I stopped using Andrea, they apparently improved the manufacturing formula/process. I have not used the new paints.