I can only think of one possible benfit of using ester based fully synthetic oil - it has good detergent properties and flows well when cold, so if you are looking to do a slow engine flush in the winter, robert's your fathers' brother, otherwise use the GM10/40, and change it regularly (I do mine every 5K because I run on LPG so not much carbon contamination, and I use the car 95% on motorways so it's not under stress), it's cheaper than milk, or the 5/30 longlife. If you don't have a trade club card, you can always either a) get one, or b) use neo brothers, I think they sell the 10/40 for 12 quid / 5L and the 5/30 for 19 quid. They are a saab agent - GM in other words.
Oil is cheap, way cheaper than a broken engine, change the oil and filter regularly and use what the engine maker recommends for use in it, they probably know best. It'll last you forvever.
I've just had much work done on mine (due to age mostly) it needed, lambda sensors, plugs, cam box gaskets (no, the breather wasn't blocked, they just got old and went hard) and unfortunately both coil packs as they got mullered by the oil that they got cooked in for the last 2k miles. Anyway the point is that when the cam covers came off I was curious to see what the cams, tappets, etc all looked like and the good news for me is that they look absolutely fine and with no significant wear, still have honing marks on. The metal surfaces everywhere were also clean, not gooey or clogged with any goo or blackness.
I also did a compression test (and cam splash feed test just to see if everything was OK - bit messy!) and all cylinders are within 2PSi across the 6, so no real problems anywhere. How many miles - 135,000 so far and it runs like new. The 5K oil and flter changes clearly haven't hurt it.
Also now I have new O2 sensors and plugs, and coils, it does about 3 MPG more at 33MPG average, and over the weeks mileage - around 700 I manage an average speed of about 60 miles per hour, so it's not idling either!
ACEA A3 is a decent spec and if it gets B4 it's about the same as the GM long life spec anyway.
Cheers
Paul