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What would happen if my mechanic (not experienced with the 4JX1) put 8 litres in when it's suppose to be 7 Litres?

First if this engine is like the older engines then undoing the dipstick tube will drain the oil to the correct amount. Otherwise carefull draining out the drain plug. Your mechanic should be doing this as all engines say fill to the level on the stick not x litres. They have an amount that should be correct but the stick is gosble.
Second are you sure the mecanic overfilled the engine? This is the engine that contaminates oil with diesel. If your oil smells of diesel youve got big problems

I am not sure. I was charged for 8 litres and the guy wasn't sure about how much to put in. He said he filled it until the dipstick showed it was full. I can't smell diesel in the oil at this stage.

Where does it sit on the stick after sitting overnight?