I have been told by numerous people the fully synth is too thin.
I use vauxhall semi-synth. The two garage I go with my two cars both use this oil. One place is a Ford and RS nut and has many years experience with the RS range. He did a comparison check on the qualities of the vaux oil & the other major labels and found they are similar - except the vaux oil is a lot cheaper (around £15 for 5 litres) as they don't have the branding & advertising to contend with.

Been using this for around 5 or 6 yrs now with no issues what-so-ever. Best thing is, as it's cheaper overall, you can do earlier oil changes for the same money.

Latest poll I did on my site came back with semi-synth 5 or 10W.
Mobil 1 was considered too thin unless it's a new build engine.

Hope this following info is of use:
My vaux oil is 10W/40 Semi-synth. Oil properties are ACEA A3, B3, API SL/CF

Properties description found via here:
Oil Bible

ACEA are here European Car Manufacturers Association
codes relate to
A3 = high performance or extended drain
B3 = high performance or extended drain


API are these people API which seem what thatcham are to alarms.
So the SL/CF rating is S= Petrol car, L = the top grade (SL = old lettering, recently replaced with SM)
CF means it can be used on commercial vehicles also