Fox used 10wt until 5 years ago then they changed to 20wt.
Apparently the 20wt sticks better to the Slider Bushings.
Many use Motor Oil and some Professional Fork Service Persons in RSA Swear by the Magnatec Fully Synthetic.
Mobil 1 is also preferred and Highly Recommended in the States.
The Problem with RS and Fox Products is that the LBS does not stock.