I'd say 10W30 is fine; I've been using it for the last 12 years in my engine in n.CA. 20W50 is used for looser bearing clearances and very hot areas like Yuma, AZ. Mobil-1 synthetic is a blend of the two types of synthetics, so you shouldn't experience seal-leakage using this stuff. It also works fine at shoot-on-sight speeds where the oil temp begins to climb rapidly in most Panteras (say, over 150mph for 10 or 15 minutes). Gives a few extra degrees of protection.
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