I am trying to find out what type of grease goes in under this cap. I think it is for the throw out bearing.
You are exactly on track. I have always just used a good grade of moly based grease in mine. 15 years now and no issues. Just unscrew that cap and fill the underlying well with the grease and put the cap back on till you feel a slight resistance. I think you were to turn it down a 1/2 turn every 50-100 hours. When you think about it a throw out bearing does not get that much time in the activated position so it doesn’t need that much lube. Over greasing can lead to excess lube being thrown on flywheel, pressure plate, and clutch. Not a good thing.