I was looking over the PQIA report on Synthetic 5w-30 motor oils and the percentage of magnesium used was the highest w/ Castrol Edge (1462) and Mobil 1 (772). The rest used calcium mainly in their additive pack. What I have read on magnesium on PQIA is that: (1) more expensive the calcium, (2) improved rust production (rust inhibitor?). The reason that I'm asking is that my VW Golf 2.5L has an iron block engine. Could this be a significient reason to use Castrol oil? Presently using GC 0w-30 which IMO is an exc. oil for this engine. Looking over Castrol TDS report on their website (which is very dated-2002 Rev 1), no info could be found on the 0w-40 oil and 0w-30 oil had .35% calcium % weight reported. (no magnesium % reported at all?) Honestly, I believe I'll only used Castrol's 0w-30/40's oils exclusively in this car for as long as I own. I wonder what Castrol's 0w-30 & 0w-40 add pack is? (Is it high on magnesium as the 5w-30?) Mostly calcium is used in all motor oil add packs that I've seen. I'd be interested in your thoughts. Thanks, Bill.