Pc are you talking about your SA200 or your 200D?
Either way in both machines the oil is 10w30.
If you are talking about your SA200 exciter arm being black, then you need to make sure ALL brushes are setaed properly and riding up and down in the brush holders. Excessive carbon in the generator and exciter housing is caused by misseated brushes. When you reseat brushes and clean the armature surfaces, run the machine on high idle working with armature rotation in front of EACH brush.
With you saying it gets harder to strike an arc the longer you have been using it, I'd have to say you have a faulty connection somewhere in the circuit. As it heats up it starts failing. Is you idle solenoid clean and dry? DO NOT oil or use anything to try and assist the plunger in the solenoid. It will only gum things up and block the magnet in the solenoid.
The F163 is an industrial engine and running the machine in the 120-190 range most of its life doesn't use a 1/4 of the power this engine has. This causes the cylinders to carbon up which in turn causes your machine to 'diesel' when you kill it. Whn you change the oil run some 'hog leg' rod on maxine. This will shoot carbon out the exhaust like fireworks.