Clutch engagement, Master cylinder/slave cylinder replacement

This is my first time asking a question. I hope it makes sense? I have a 1998 Chevrolet S10 two wheel drive 2.2 liter, with five speed, with 103,000 miles. Original transmission, clutch and hydraulic parts. My clutch feels fine most of the time with good pedal pressure and easy shift when cold or stopped. Lately when approaching a stop sign/light, the truck stays under power when clutch pedal has been depressed like it is failing to engage. Pedal play is good but at times seems to be weak. I have found no leaking fluids except very small amount of fluid or oil that appears where the rear main might possibly be leaking but fluid in the clutch master cylinder has remained the same level. Could a small amount of motor oil make the clutch slip and not engage? Clutch master cylinder fluid very dirty, black and filthy, like DOT-3 has some type of liquid swirling floating on top. No drops of any oil or fluid detected on driveway, floorboard or engine compartment firewall. The rubber flexible part of the clutch master cylinder cap(interior) is pulled down into the fluid not up in the top of the cap, like the brake cylinder caps bottom interior is positioned. Should it be pushed up into the bottom of the cap top or down into the fluid? The fluid level is down approximately one to one and half inches from the top of the cylinder top when the cap is off. If the cap bottom should be up into the cap not down then fluid could be added. No leaking fluid in the floorboard or engine firewall. At times it is becoming hard to shift into first from reverse or neutral but when accelerating through third, fourth into fifth I rarely use the clutch. I am considering changing new DOT-3 fluid in the clutch master cylinder, is that possible, would it help anything if it is contaminated. Could it get contaminated during normal use? How could it get so dirty unless a seal or something was failing in the clutch master or slave cylinder? Should I go ahead and change clutch master cylinder? I always thought the sign of a master cylinder failing was fluid leaking under dash or on the firewall around master cylinder. I am unsure where to start. Could the master cylinder be losing pressure at times then working at other times? Is there a procedure to test the clutch master and slave cylinders without dropping transmission bell housing, etc?
Thank you for your time.