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Magnesium was probably used simply because aluminum was at a premium when the car was designed. It'll also burn like a sparkler if you get it hot enough. The later typeIV was aluminum for a reason.

Walter the aluminum cases are much stronger than mag. Most of the current top vw racers have changed to aluminum and run there cases for multiple seasons. The thing to understand is that when vw went to an aluminum case for the type 4 bus engines they had to make changes to the engine they increased the size of the oil cooler and also increased the cylinder head fins by 40 percent to increase cooling. But it is very rare to have a line bored type 4 case and almost routine to line bore a type 1 mag. case. There are many things that can be done to help lower temps. #1 make sure the cooling passages in the cylinder heads are clear of casting flash. #2 switch to a type 4 oil cooler and #3 paint you push rod tubes with high heat white paint (this can lower oil temps 10-15 degrees) Also wrapping the exhaust ( if its stainless) with header wrap will help. Hope this helps. Scott

I thank every one of you for your advise and Scott, everything you mentioned with the exception of white paint, I used red paint, is done.
what are your CHT on climb out and then at cruise at 3000 rpm at 3000 feet?