its always better to run fully synthetic in these cars, the oil doesnt break down at higher operating temps like semi does. the the engine was designed to run on semi it because fully syntetic wasnt as available back in the early 90's...or so i've heard.
i've used castrol rs, agip and millers 10w60 and of the three millers definately seems better, the engine sounds smoother.
and change it at least every 3000miles