Your 50:1 fuel is made up of 1470.6 ml of fuel and 29.41ml oil.
To get to 40:1, you need 36.76ml of oil. This is an increase of 7.35ml.
There's a useful function in excel called goal seek. Find your original individual amounts by putting 1500ml total, with your amount of fuel equal to 1500ml minus the amount of oil. Then you set another cell as the ratio of fuel to oil, which in this case you want 50. So you use the goal seek function to tell the amount of oil cell to change in order to achieve 50 in the ratio cell. That tells you 1470.59ml of fuel and 29.41ml of oil.
Then you do it again, but you leave the amount of fuel the same, at 1470.59, and change your total to a formula of that plus amount of oil. You then goal seek on the oil amount to make the ratio cell 50, and the oil amount ends up as 36.76ml, which is an increase of 7.35ml.
The 30ml of oil asserted above gives a 1:49 ratio, and also doesn't account for the fact you have an increased overall volume once the oil is added. I'm splitting hairs I know.
