Just got my first slant six

Ditto on the Schraeder valve. Never replace with a normal one for a bicycle or car tire. Those have a very strong spring (black tire). You want a special one with a very weak spring. I bought some long ago at an auto tools specialty shop. They have white plastic collars, which must identify them (AC ones are green).

If the OP puts solvent in the bottom of the oil pan to de-gunk the pickup screen, then drains it all out, that shouldn't hurt the engine, if the engine is never cranked. But, better to wash down all the top-end crud into the pan, then remove the oil pan to clean the bottom side. You can work on the oil pickup then and even change crank and rod bearings from below, along w/ rear main seal. Most likely, the pan gasket needs changing anyway.