Is the oil pressure just dropping at idle/low RPM's? If so, then thats normal as the engine warms up. When i've been on a long journey and my engine is at full operating temp, I get about 1 bar at idle, but this pegs 3 again as soon as i'm cruising again.
In a performance orientated engine like that, maybe a 5w 50 is a better choice if you have frequent cold starts. I run a 15w 50 in my Porsche, but I only use that in warm weather.

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Ben

'85 500 SEL
'85 928S2
'88 560 SEC
'90 750i