I have used them together many times Wooty. As long as bith bottles say enamel on them, you should be able to use them together with some degree of success. I am pretty sure that the Testors Enamels Thinner is the same as Model Master Enamel Thinner.
There will surely be those who disagree with me, but I use mineral spirits whenever possible. There are times, such as airbrushing, that it is smart tot use the manufacturuer's thinner but I will always try a cheaper, larger volume alternative. Experiment with different combos Wooty and find what work well for you.
US ARMY HELO DRAB is a great starting point for Army H-60's. We just had our aircraft repainted before we deployed and it matched the fresh paintjobs very well. To lighten it for weathering you might try adding a little gray and bringing out the olive. On a weathered aircraft, oil on the paint looks like new paint with a sheen. Remember that this is a what some refer to as a dull finish meaning
very flat