hey mate, i'm no expert on this but since no one else replied i'll have a crack. A fully synthetic oil would provide better lubrication etc but for ecotec's it has to be the correct velocity, holden recomends 20w50 so as long as its not too far from this, i think a fully senthetic oil would be better, but i'm sure someone else has a better idea.
The last couple of pages you will find the most useful. As far as viscosity goes, Holden recommended 20w/50 when the Ecotec came out in 1995 because it was the best viscosity at that time. Oil is continually developed and improved each year. In 1995 the best API rating was either and SG or SH (one or the other. Then came SJ, SL and now SM is pretty much common.
I use a 10w40 in mine. It's better than any 20w/50 in 1995.
As far as semi vs fully, Some companies claim their product is fully synthetic but actually made from mineral base oil. As long as the oil is made from group 3 or higher oils you'll be right. These oils will resist breakdown better than an oil made from group 1 or 2. Look for an oil with an API of SL or SM and an ACEA of at least A3/B3.
One recommendation I can offer is have a look at Repco branded oil. It's actually rebottled mobil. I used their semi synthetic 10w/40 because I needed a cheap quality oil until my next proper service after changing a LIM. At $32.95 it's great value.
Also read the posts made by vxcalais_01 in the above thread. He knows more than most about oil.