Hi JPCARE,
I don't think anything is too soon after a long hiatus. I just completed my first build (Me-262) in over 25 years. Next I am doing a P-38 with NMF which I am probably not ready for. But I'm gonna try it and see what happens! As long as you're having fun, go for it!
For oil stains on an aircraft, especially a wartime bird in hard service, my personal opinion is to minimize wet glossy drips and runs. These will probably be wiped away by ground crew, leaving behind a smudge or streak. Or when the plane takes off, the high velocity air flow will spread it into a fan shaped pattern. Having said that, I recently went to Planes of Fame in Chino CA and the flight worthy birds all had visible oil leaks with large drip pans underneath to keep the oil from staining the concrete floor. The leaks were mainly visible at the lower surfaces of engine cowlings, nacelles, and on the engines themselves. Idk if black would be the right color, up close it looked like a very dark translucent brown or amber. But on a model, black might look better. I don't think there is a right or wrong way, try different things and use what looks best to you.
For wiped and wind blown oil stains and smudges, I use a mixture of post shading, washes, and Tamiya weathering master. Also, an HB pencil applied to the leak point, then wiped in the direction of airflow, can be really handy for smaller stains and smudges. Again, there is no right or wrong, try different things and use what looks good to you.
Good luck, and have fun!
Edit: I looked at the photo in your OP and I was way off track. I think for that look I might try an oil based paint. Maybe followed by an acrylic clear coat.