As Pete says, you must check the oil on a warm engine. If you don't it is highly likely you will end up over-filling, which WILL result in oil loss as the excess will inevitably get blown out somewhere (usually into the airbox).

What happens is that on a cold engine a fair amount oil drains down into the sump. After warming up the oil pump will scavenge this excess and return it to the tank, reaching an equilibrium. This is the point at which the level should be measured.

Have a read of this.

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