Gear oil is thicker than engine oil but is diluted with diesel, Diesel is pretty inert at low pressure, you can throw a match into a pool of Diesel and nothing will happen (except the match will go out).
Perhaps additives are good but it costs £10 per litre, Diesel is cheaper
About 2 litres of gear oil and 5 litres of diesel and run it through. Obviously without starting it, it's just for running through, the gear oil will protect the valve stem seals etc