This is where the focusing screen is (ignore the magnified section)...
Never use cotton buds to clean the inside of the camera. If you want to clean it you ought to buy some proper cleaning products - they don't have to be expensive and you won't spread dust, oil or solvents around the inside.
Get yourself a proper blower (£8), some PEC pads (£10) and some ROR fluid (£4). Use the blower to clean out the inside of the camera (around the mirrorbox), drop a small amount of ROR fluid onto a PEC pad and carefully wipe the focusing screen.
Would it be safe to use the POR fluid and PEC pads to clean the actual sensor?
Ive tried surgical swabs and a little Vodka in my old Dynax 5D, did no harm, but I thought after it was a trifle stupid.
You could certainly use the PEC pads on the sensor - it's what they're designed for.
I don't know about using ROR fluid on the sensor. It's primarily designed for lenses and there is no information on the Opticron website about using it on the sensor.
To be safe, I'd probably use PEC pads and Eclipse fluid which is designed for lenses and sensors.