Thanks for all the comments guys,.

I use a 70W which is quite viscous and not a problem, not super quick but okay and at least your not lying under the car dripping oil.
If concerned you could always put the pump in a bucket of hot / boiling water fro few minutes which will heat the oil and reduce the viscosity.

It is all done with air pressure so there are no moving parts to be effected by viscosity, only the friction losses in the hoses will increase with thicker oils.

p.s. High viscosity = Thick Low viscosity = Thin / runny.


Cheers,
Richard B.