I have a 99 2wd sierra with a 4l60 in it and I was changing the oil in it and decided to check the tranny fluid level and it was fine but the fluid was noticeably darker than it should be but it doesnt smell burnt or anything the tranny itself shifts fine and doesnt slip should I just drain it change the filter and fill it back up with fresh atf and go or should I be lookin into gettin it rebuilt