Thank you for the reply Phil please don't think I'm picking at your method I'm not, I'm just trying to pick up as much data as I can.
I was an egineer before I retired although competent on a good day was about my best effort.
If you are interested go to the 3rd post down on this page from the top where I listed Walther's Equation, but more to the point is a link to what (IMHO) is a pretty good paper on testing and gives a breakdown of the various calculations that can be used to interpolate (or extrapolate) oil viscosity at given temp
Many thanks for your efforts and your replies it's appreciated
Hi Old Jock
Not at all, your thoughts are very interesting, I just haven't got the patience to learn a new formula (not yet anyway)!