I prefer the first one, but if you really must, you can use "a little" (no need for "of").
I like the first because "amount" because it sounds like someone has deliberately measured the right quantity of solvent, whereas "a little" sounds like someone threw it in without measuring -- of course if that's what they did, it's fine.
I thought it was always "a little of something" and "a little" is an adverb.
Here are a few lines from magazine <Esquire>:
"And when you stand up straight, announce what you want, and show what you are willing to give -- a little of your time, a measure of your good humor, a moment of real attention. That directness is accepted as style, that clarity as power. That's when the giving becomes the getting."
But why "a little of solvent" is incorrect? Thank you.
I'd like a little of your time. In return, I'll give you a little of this, a little of that, maybe even a little of what you fancy.
But I can also give you a little love, a little tenderness, a little advice, a little solvent, and maybe a little piece of my mind.
I think I'll let someone else give you the details. I'm just the mechanic here. While we're waiting, I'll suggest that if you search for "a little of" in Google, you'll find that it's only used in certain phrases that are pretty easily memorized.