Why does my oil get dark!

As you can tell from the title my oil seems to get dark alot quicker then it should be. Since the rebuild the first 2 oil changes did it and i figured it was normal from the assembly lube getting in the oil and just still being broken and stuff. Even right after a fresh oil change i can see the difference in shade after like 2 weeks. The weird thing is if i check it as soon as i turn off the engine its nice and clear but if i check it after the car has sat for a few hours its alot darker?? BTW i had just switched to Mobil1 synthetic after 2,000 miles and still does the same thing. If it matters i took off my upper and the ports look wet, i dont no if its gas or oil? and i also have a see through inline glass gas filter in the pcv line but it doesnt look gunked up. Should i whip out the leakdown tester to check for any blowby.

Speedystang, theres no smoke at startup nor do i see any at WOT.Also there new positive type valve seals also, The only thing i have is a weird smell on a cold start for a minute but im sure thats from not having cats. I agree with you Hissin on how some motors would like to dirty up some oil and some not, but i had this block completely stripped, i took out all frezze plugs, oil gallery plugs evrything,clean everyhole with a oil gallery cleaning brush, hot tanked it, steam cleaned it, after i bored and honed it, i steam cleaned it and all once again to get all the shavings and honing oil out. Its practically a brand new motor, i just dont want sludge building up from having grimy oil, ya know. Ill do an old school trick by taking a lighter to my dipstick to see if theres any gas in my oil also. Just in case i might do a leakdown test if i can get access to shop air and/or ill also do a compression test, just to check if theres anything fishy going on in there lol