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911T oil pressure lowers when warm

Engine

Vehicle Information: Model: 911T; Year: 1969; Body Type: Coupe; Engine (type, size, modifications): 2.0T with Weber carbs; Total Mileage: 132,000 +;

Have recently purchased a 1969 911T, but unfortunately don't have the history of the car. Engine runs strong across the rpms, and pre-purchase leakdown test was between 4-6% across the cylinders, suggesting a recent rebuild. The problem I'm seeing is that when driving with a cold engine (below 180 degrees), the oil pressure reads reasonable from what I've read (50-60 psi at 4000 rpm), and the oil level gauge reads low - both normal I believe. As soon as the engine temp hits around 180 degrees, the oil pressure slowly starts to drop to a fairly consistent 20-30 psi (drops lower at 1000 rpm), and the oil level gauge starts to read full, as if the car were idling. This occurred during 10+miles of hwy driving at 55mph; after noticing the problem kept the rpms between 2-3k. Appreciate any thoughts, as my '58 356 isn't nearly as "complicated" :-)

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