oil gets dark really fast
 

My oil has been turning dark brown very after 2 days after the oil change. Before it usually lasts a week before turning brown. I use mobil1 super synthetic 5w20 which is a very bright gold color. I buy the big 5quart and usually use 3/4 a quart to drain out the rest of the black used oil. When I check the level its usually a goldish brown after a week. My last two oil changes when I checked after 2 days its already a pretty dark brown.

Has anyone had this problem? Thanks for your point of views.

RX-8 oil change leaves about 3 quarts of old oil in the system. So your 3/4 quart "flush" still leaves a significant quantity of old oil in the system. It mixes with the new oil, and Voila!, dark oil.

Two days is about right. Keep in mind that when you drain the oil, only about 50-60% (maybe 70% if you do it right) of the old oil is drained. The rest stays in the oil coolers and oil lines. Total dry capacity is about 7 qts. If you try real hard, you can get about 5 qts. drained, leaving 2 qts. of dirty oil in the system. It doesn't take much mixing for the oil to turn dark.

Even with new engine/car, the oil inside gets dark as fast as a week...

if you perform oil changes as mazda specifies in the manual (every other fill up), it should not turn dark.

reading comprehension FTW!

Oh, damn. Thanks a lot guys.