Fork oil is directly and proportionally blendible
so 50% 10 + 50% 20 = 15
SO go with some light oil and blend up
Buy a bottle of the heaviest oil you can find and the litest oil you can find.
Run kerrosene ( parrafin to some ) through your forks few times to clean them out.
Thus you can drain the first " test" out the adjust it and return it to the forks till you find what you are happiest with.
Use genuine fork oil , not engine oil.
Fork oil was not invented when the bike was made and is head & shoulders better than engine oil or ATF in forks.
IT contains preservatives for the rubber seals, water isolating & anti corrosion addatives not found in any other oils
