I would NOT suggest a guy new to motors attempt to read and interpret the results of a micrometer reading on crankshaft journals. This assumes he even HAS a micrometer!
Wipe the
oil off the crank journal, bearings, rod surface etc. so it's clean and dry as possible and use PLASTI GAUGE to take a reading.
Low pressure could be a HUGE number of things, like bad\worn cam bearings, which you CAN'T check. I'd PLASTI GAUGE the main bearings, as they are more suspect than the rods actually. I would NOT check the rear main bearing for fear of upsetting the rear main seal. Which may not be leaking at all.
Oil on the bottom of the pan, rear, could be valve cover or as mentioned, rear of the intake manifold.
And don't get me started on PLASTI GAUGE, if your not using it I wouldn't let you near MY motor! DO take your mic readings, do the math, calculate the clearance and CONFIRM the readings using Plasti Gauge. What? Your SO good there is NO WAY you could possibly make a mistake doing the math on your Mic readings? And pigs fly!
I won't take chances, I'll double check, only way to be sure.