If you see it in the VF, it's on the focus screen not the sensor (no effect on images). Question is, dust or oil? First step is to simply remove the lens, and with the camera held mount-down, use a rocket blower in the mirror box. Dust should just blow off the focus screen. If it's oil, send it in - the focus screen is easily scratched. Alternatively, a new focus screen is $40 in the US. If you go that route and it keeps happening, it'll need to go in.
Hope mine doesn't do that. I took it in (in person) last week for the lubrication advisory, had dust on the focus screen and sensor, got it back an hour later with both clean and the recall issue (hopefully) addressed.