what the nissan mech told you the noise is I'd agree. I tried several different oils and grades and dependant on where you live and the ambient temps you can up the viscosity rating to say a 20w-50 in the summer and a 10-40 in the winter. My lifter tap went away about 95% using a 20w-50 synthetic oil. You need to understand what is happening in the lifter to understand why the rislon or a thicker grade of oil is silencing the lifter. There's a ball that acts as a check valve retained by a spring inside the lifter. Due to dirt or debris (lack of oil changes regularly) that has circulated in the lifter the ball has been srcatched or worn which allows the lifter to loose it's prime, which is the oil that's retained inside the lifter to act as a hydraulic dampening medium. Once this has occured there is no fix other than changing the lifters to eliminate the tap. By using a thicker oil or additive that thickens the oil you are effectively sealing the worn lifter which is why the noise goes away for a while then comes back eventually. Hope that simple explanation helps.
