Measuring paint and thinner to mix

I just eyeball it.  I use Testors enamels.  I find the mixture ratios not that sensitive.  If I get it too thick, it just will not airbrush well.  I just add drops of thinners till it does.  If I go too thin, and it tends to run, I dial back the flow so I can get a thin base coat on.  Once I have that base coat, I can use the thinner mix more easily.  However, I can also just add a bit more mix to the jar.  By avoiding measuring, I avoid having to clean the mixing instruments.

I use the "spoon" end of Tamiya stirring sticks to measure paint. They are made from metal so a simple wipe with a rag cleans them off. I use plastic pipettes to measure thinners. I have great control of my drops to get exact ratios using both of those tools. 

I have found that turpentine/artist thinner and oil based products swell the rubber plunger making it hard to get the fluid out of the syringe. Normally have to throw them out if I use thinner for weathering.

I use these plastic pipettes  

They work well with lacquer, enamel, and acrylic paints and thinners. I bought a 500 count about two years ago, and I've used about a quarter of them. The clean up is really easy, I just throw them in the trash after they have been used. 

Do they dislike oil for the first use or is it just the 2nd or 3rd use when they have issues?

I use a set of glass eye droppers that are marked with graduations of .5 CC to transfer paint and thinner to either a 10 or 25mL graduated cylinder for mixing. The graduated cylinder makes it very easy to pour into the airbrush and, since everything is glass, clean-up is a breeze. The eye droppers came with bulbs in different colors so that I can designate one especially for thinner to avoid cross contaminating. You can find all of that in the medical and scientific equipment area of Amazon at really reasonable prices.

I use disposible measuring syringes for acrylic and thinner. They are cheap on ebay and reusable. But they tend to dislike oil based paints and thinners.