I have been working at the dealer since these engines came out. I seen very little lifter problems with them. Here are a few thing That I do see that cause lifter noise:
I seen the cam lobe wear and cause the roller on the lifter to wear and cause a noise to start with and then start to cause a miss at idle. You can pull the valve covers off and attach the coils so it will run with the valve covers off and put a little pressure on the lifter side of the rocker arms until you find the one where the noise goes away. Then you can tear it down and check that lifter. The Cylinder head has to be removed to get the lifter out.
Seen the heads get a very small crack in them and cause a little coolant to get in the oil and cause the oil to thicken up and cause lifter noise. This condition will cause it to use a gallon of coolant in 3-4 months.
Seen carbon build up in the cylinders cause tick in the engine that sounds like lifter noise.
Oil pressure can get low at idle and cause the lack of oil to the lifters. Cam bearings can wear and cause the low oil pressure.
Cheep oil filters
can cause the low oil pressure. When you change the oil, make sure you use a good quality oil filter.
At this point, with the" lifter noise" that just comes and go, I would start out with an oil treatment for lifter noise first and see if that helps. Due the the fact that the heads have to be removed to get to the lifters, I would wait until it get more consistence until I would tear it down. You could use a heaver weight oil in the summer to see if that will help. At this mileage, you may want to use a heaver weight oil anyways to make up for any wear in the bearings.
Hope this helps
Let me know if you have any more questions about it.