Ok, we are on the same page.

The way I read the sentence to begin with was

Originally Posted by vint_mow
Think I will go with Tyler's suggestion of 40 to 1 using a modern semi-synthetic (ByNorm) and see how she goes.

hence the literal 40:1 is equivalent to 25ml to 1 litre. 100ml per 4L or 125ml per 5L