.... although ive had to top up the coolant about 4 times in the last 3 months.
Sounds like it's been on the way to Boot Hill for a while and finally got there...
When I changed my H/gaskets recently (v6) the rear one had some serious delamination and a collosal split between c1 fire-ring and the water channel next to it..
I also looked at the gasket on the front bank and I could see some evidence of cracking in the face of the gasket where it was exposed to the coolant and signs that the coolant had started to make that gasket start to delaminate too.
It seems to me that over time the gasket material cracks, which lets coolant in between the laminated sheets of the gasket.. rusts the metal insert (or the coolant pressure forces the layers further apart) until the de-lamination reaches an oil way or the fire-ring.. which obviously allows coolant, oil and/or gas to mix ... Game Over.
Dunno why the gasket face should crack.. it could be that coolant seeps in somehow and then forms vapour bubbles, or rusts the metal layers causing the material to expand slightly and split...
I've always used anti-freeze too.. which is supposed to prevent corrosion... but my gaskets showed clear signs of rust (aluminium engine.. though some of the pipery is steel, admittedly).
With my new ("Victor Reinz"
) gaskets I've used a 60% concentration of the synthetic (pink) anti-freeze and distilled water only (no salts)... as the 40% mix of the blue stuff and tap water obviously wasn't keeping corrosion at bay (and I used to change it every year too...)