Do not exceed about 6% or so of oil paint per mixture.
Yes, I use D-Limonene on my last mask and I have seen others use Naphtha and Bare Mineral Spirits to thin it, though I'm not sure which is best, or what definitely works. But D-Limonene (the citrus stuff like what they put in the hand cleaner etc) is what I used.
use a small cup, mix some Gel10 in it and mix in your pigment / paint, switch to a new cup and put your thinner in it DO NOT USE FOAM CUPS, the solvent will destroy it! It's a solvent so it'll dissipate and leave your pigmented silicone.
If I get an extra moment I'll find my saved videos and post them of what I used for references
PS. Using SILICONE PIGMENTS like what Smooth-On and BITY sells, is much, much better. See this guy's head in the video? See how translucent it is? It's TOO transparent. Use real pigments if possible and flocking. You'll never go back! It just takes a tad, BITY has a very, VERY affordable pigment trial tray that has like, 6 colors or so all in an acrylic base, has all you need. Order that and the red flocking and you're set. The better you pigment your silicone, (not to overdo it tho) the easier it is to paint. This guy here, his paint looks very pale, that's because of the head being too translucent.
Hope this all helps.
Btw, this is a reference video I used, this is not me or my video.