bit of a headache

my kawasaki 260x has started to use engine oil

i have had to top the engine oil up ( 300ml ) to the correct level on the dip stick , using 45 / 50 litres of fuel

this has happened on the last 3 times, out on the ski , each time using about the same amount of fuel , then topping the engine oil up 300- 400ml of oil each time

i have owned the ski for 5 years
the ski has 135 hours on the clock
the ski is only used in fresh water lakes
the ski is completely standard
no loss of power

is this a engine out / rebuild job

i have not done a compression check yet