I have done this more than once. With my last toyota truck w/4cyl engine. I ran premuim Shell fuel. I used castrol GTX oil change every 3-4k miles. Oil looked almost like I put it in. This continued for 55-60k miles. Then I started using mid/lower grade Shell fuel-the oil was darker to very dark. I had done this because of expenses at the time -and I figured no big deal.This was for about 20k miles. At approx 85-90k miles I went back to premium. After the 2nd or 3rd oil change oil was once again much like fresher oil/honey color. I owned the truck up into the 210k range before selling. never burned a drop of oil. Oil was still looking same color.I contribute to better detergents in the fuel (guessing) but I am convinced my engine stayed cleaner/ran cleaner because of it. My wifes CRV had same oil @ 3k changes w/premuim fuel. Her oil always looked like I just put it in. My daughters Civic oil was always black at 3k miles-same brand oil but low grade fuel. All 3 of these vechicles were bought new. Just an observation. BTW I used most all that used oil in my chain saw bar oilers for our fire places/wood stoves.