STOP! DO NOT DRIVE YOUR SAMURAI ONE MORE FOOT!
Your fuel pump is dumping fuel into the crankcase and, in your case, is LITERALLY washing the internals of your engine! That is why your oil looks dirty - even after a short time of operation.
As the fuel thins your oil, the oil loses it's ability to lubricate parts properly and suddenly your engine (crankshaft bearings, camshaft bearings, piston knuckles, EVERYTHING!) starts to disintegrate from excessive wear!
Check your dipstick for the "appearance" of more oil to prove that you have a fuel pump leak.
You need to IMMEDIATELY replace your mechanical fuel pump. Be VERY CAREFUL when installing the new one that you do not bump the pump lever on the back side of the pump and cause the new fuel pump's pumping diaphragm to be damaged.
You should turn the engine over CLOCKWISE by hand (use a 17mm socket on the front of the crankshaft with the sparkplugs removed) until the fuel pump pushrod retracts into the head before installing the pump. You will have to push the rod in as you turn the engine as the pump lever pushes it back when installed. This will eliminate any pressure on the diaphragm by the pump rod as you install the pump.
Also, DO NOT BLOCK THE BREATHER TUBE WITH ANYTHING! Doing so makes the pump work harder (and thus causing damage again) as the air behind the diaphragm has nowhere to go when displaced during each pump stroke.
See the following link for a pictorial:
Ack's FAQ Samurai 1.3 Mechanical Fuel Pump Symptoms - Gasoline and Oil Leaks at Vent Tube
Also, ALWAYS change your oil filter when you change the oil. It's less than $15 (possibility less than $10). Doing so removes all existing dirty oil while giving you a new, clean filter element. Oil changes shops do it, not to rip you off, but to actually make your engine run longer - not to mention that the owner's manual reccomends it too.
You can't imagine
how much I hope that this helps you!