2008 Pontiac Solstice GXP
I did a simple non scientific test with clean gas and it took about 2 teaspoon of cheapest harbor freight black grease to match the color from the first test.
Think about that for a second. Oil is not black. oil is not opaque it has some transparency to it. If you put oil into the gasoline how much do you think you would have to add to get it to the color after you flushed it.
As a suggestion.,...
It would be better if you used kerosene instead of gasoline, it dissolves oil deposits better then gasoline does. I would fill the intercooler with kerosene to the top and cap the pipes, let it sit overnight. next day uncap one of the ends and pour out 2/3rds of the kerosene into a container. then recap and shake the snot out of it, uncap and in another container empty the intercooler. Pour in half of the kerosene that you dumped out initially and shake. do the same with the last amount that is left. I bet you get a whole lot more out, possibly some chunks will come out as well. You will want to hang the intercooler so it can drain, let it hang for 2 days or so before reinstalling.