Hi all...I have just about got my 1949 ZM20 bike ready for a road test, however when I took her off the jack the forks compress excessively. The strings are good. I did fill them with 142mm of 30 SAE oil in each fork leg from the top nut. Should they be filled with oil another way? When I drained the old oil out I just turned them upside down, so assumed they were filled from the top.

Ron should be sble to give you the answer. Lots of people have told me he knows fork oil.

The oil in your forks has no effect on the ride height, this is controlled entirely by the installed length of the springs assuming all other components are standard. It is fairly common these days to come across older bikes with "modern" springs substituted for the correct ones often including additional spacers. The correct are available from several sources.
Richard