Daaa go flush or please flush
It depends on what type of flush you use. Old style single flush toilets use around 11 litres of water per flush, older dual flush toilets use 4.5 or 9 litres per flush, while modern water efficient dual flush toilets use 3 or 6 litres per flush.
In a modern toilet you use 13 litres per flush.
I would use my shrink-ray and then flush it.
New toilets almost everywhere are 6.5 litre flush.
You will not damage the engine but you can damage the radiator if you use any high pressure. If you only flush and apply no pressure you will do no damage. Use a chemical flush and a water hose and you will be safe.
low flush toilets use less water.
Urinate and/or defecate in it, then flush.
You always should after you use it.
Dual flush toilets usually use 3 and 6 liters of water depending on which way you push the flush handle - 3 liters for a little job and 6 liters for a major event. Single low-flush toilets use either 4.5 liters or 6 liters.
One flush uses less water. It saves water when you just want to flush down 'number 1', the yellow stuff. If you want to flush 'number 2', then you use the stronger flush (or what would be the usual flush in North America). Basically, it's ecological.
About 1.6 gallons per flush