Not that its the problem,but 10w-50 Like gear lube at start up and like water when up to temp or is that backwards? The short block was done at the machine shop,then the machine shop needs to find out what is wrong.Did they put the oil pump on or did you?and which oil pump did they recommend and what bearing clearances did they use?The block may be set up for racing and have to have hv hp oil pump.