The oil would seem runny because its diluted with petrol. Try standing an open pan of freshly drained oil in a breeze-free room and see if the surface of the oil seems to shimmer slightly. If it does, that's the fuel evaporating. Fuel dilution might also be indicated by the oil level in the sump apppearing to rise (called "making oil") but this might not necessarily be so if the engine is burning oil as well.

Penzoil (and I'm sure others, too) makes a good 'first fill' oil for new engines. 'First fills' are good in pressure applications (bearings etc.) but relatively poor under "shear" (sliding), so allowing easier ring bedding in while still adequately protecting bearing surfaces. Highly recommend their use.

Another possible cause of your problem might be that one or more of the compression rings have been fitted upside down. That's impossible to check without dismantling, so I'd try running the engine in properly first!

Cheers