2001 Chevy Silverado Tapping/Knocking Lifters

Did it tap before you put the synthetic oil in? But I think its just the cold weather letting them tap before the oil gets to them. The lifters are hydralic lifters and there is no adjustment for them. The oil pressure sets them. Does it quiet down after a minute or two? If so then its just that the oil hasn't up to them yet. The oil returns to the oil pan when you cut the motor off and when you crank it it has to be pumped back up to the top of the motor.