I thought I'd throw my 2 cents worth into the ring, and perhaps start the argument all over again.
After suffernig from diminishing oil pressure during extreme competion use in my Cooper S about 10 years ago, I contacted Penrite and asked for their recommendations.
I was using HPR 30 or 50 (depending on conditions) and it was fine except for long races in hot conditions on a track like Philip Island where you sustain high revs and full throttle for long periods of time. Oil temperature was extremely high even with a cooler, and therefore it was unable to maintain viscosity and pressure.
The recommendation was to try their 'new' 10 Tenths 25, competition engine oil, which is a synthetic with a 25W-60 rating. I was given 5 litres to evaluate and it did the trick. The engine is only 1293cc, but it revs to 8,000rpm and shares the oil with a straight cut gearbox and Quaife LSD. Since then reliability, or wear in engine, box or diff hasn't been an issue.
The oil is expensive, but I am so impressed with it that I use it in 2 other competition vehicles as well, one of which runs a small-bore A-series engine.
The general consesus in this thread seems to be away from synthetics because they are too light. This one has a similar viscosity to the other oils recommended here and because of its synthetic nature is more resistant to breaking down under extreme heat. And Doc, it doesn't leak from any seal or gasket joint.

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Curly
'64 Cooper 'S' (Group Nb spec)
'67 Mini Marcos GT
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