The ETZ 251 manual says 230 – 250 cc, 350 to 370 mm high. Measure the height with a bit of rod right down the middle of the fork. They also emphasise it is important to get both forks to the same level. As for grade of oil, I wonder if the spring is already a bit stiffer to take account of the extra weight. You may need to experiment. I got very good at swapping forks around when I was testing three sets to find out which ones leaked.
Keith