The point I was trying to get across was to get a bottle of the heaviest fork oil you can find.
Usually that will be 40W from a BMW shop then add a little and a little more till the forks feel comfortable.
Take a note of how much you have added, do the math better than me and that is the oil you should be using.
And a W after a number is the viscosity index.
A single number ie 40W is 40 at Standard temperature & pressure which from memory is 20C and seal level in the Med
A second number ie 25W 50 is 25 at 20 deg & 50 at 100 deg.
Where it get confusing is the second number is the measured viscosity of that grade of oil at 100 C
SO the oil does not get thicker when it gets hot, it gets thinner, but it thins to be the same as 50 would be when it is 100 deg.
Because fork oils should never get to be 100 deg C fork oils are generally mono grades