don't put straight synthetic into your 16 year old ca20 that has been running on mineral oil for its whole life. It will screw it up. The best you can go for in terms of quality would be a good branded semi-synthetic.
Now, in terms of W number this is the oils viscocity when its cold and at operating temperature. The lower the number, the 'thinner' it is.
As with everything in life you get what you pay for, but it "is" only a ca20 and I assume you haven't worked it off its tits? In which case pretty much any brand oil of the right viscocity will do the job, as long as it is replaced at intervals no longer than 10,000km.
In terms of the different types-
a straight mineral oil will suffice for engines that aren't thrashed and/or highly strung/modified
a semi synthetic is good if you want an oil that can stand up to some abuse ie for track days or if you cain the crap out of your car on a daily basis.
synthetic is the best obviously, however I have been told that and engine wears into a certain grade of oil, and you shouldn't go straight to synthetic otherwise things will break. Dunno why, just what I have been told.
I used to have a good website about all things oil....but its lost. When I find it I'll post it up here as its got some great info on oil stuff.
EDIT HERE IT IS:
http://www.chris-longhurst.com/carbibles/engineoil_bible.htmlI'd go more off of that website than what i have told you!!