The "full" is 20 quarts, something near to 19 quarts, a bit less.The right engine can store up to 2% of more oil than the left one due to engine position.Engine quantity for refill must be checked in the correct period of time or you will refill it wrongly.The period is between 5 and 30 minutes from shut down. Not before 5 minutes, not after 30 minutes.

The manuals are straight from Boeing - if there's a mistake, it's theirs.

The only reference to quarts in all the 737 manuals PMDG supply is on page 0.00.68 of the Introduction.

For technicians is better to use quart.One oil can is 1 quart. (Yes I know, is like a liter, but they are not exactly the same)

Only if your can is a one quart can and your quart is the same size as the quart being talked about. Where ever you are in the world, a liter is a liter, and 737's fly everywhere in the world.

The oil can you see (from Mobil) is the standard can used worldwide, all are in USG quarts, except if bigger (gallons) can are used.As the oil must be used when can is open (and the remaining oil must be discarded) quarts are what is used.For pilots there is no difference in what unit is used for oil quantity, they must know maximum and minimum values, then, the display could show apples or numbers, dots, or dashes, ** or faces, the important thing is that what is shown reflect the engine condition and that limits are known.As told before, as the quarts are the default oil can you find everywhere the display is set to use them or the percentage.If you use a liter can you can use it, the % scale is better as the tank is 20 liters, 1 liter is 5% while with quarts there is an approximation.