If your car feels fresh after a service then I reckon its probably due to cleaned/replaced ignition components more than the oil.
Renault recommend 10/40 and 15/40 semi-synthetics for the Williams. Im not one to argue with that and the Renault specialist I use always fills the engine with Renault branded Elf oil. Im sure its not the most technically superior, but I think that most branded modern oils are way superior to what we used to have available anyway.
Nice to hear of a cared-for low miler. Mines got 62k on the clock, but it does keep creeping up! If the car is new to you, watch out as I found that although mine had low miles when I bought it, a lot of the rubber-based parts had worn just as quickly as on my old 16V with 105k on the clock (steering column parts, anti-roll bar mounts etc.).