no oil pressure at startup...possible synthetic oil?

[jetfiremuck]

Im finding no oil pressure upon startup after sitting for a few days. itll run for 2 minutes before the light goes out. Im running 15-40w full synthetic. I did a mechanical oil pressure test and its reading 60psi cold. The engine has just had new mains and big ends installed and has over 70 miles done since refresh.

1. Is it possible that the slipperyiness of the synthetic oil is causing drainback thru the pickup tube into the oil pan.
2. Ive removed the oil press relief valve and spring and checked all good there.

3. Ive run the engine since refresh using synthetic oil.

Crank wasnt removed for bearing changeout. Oil pump was reinstalled and primed .

Obviously this cant go on without bearing damage. If i idle up the engine to 1400rpm for a few seconds or so light goes out. Shut off and restart all ok until left overnight.

Any ideas.