IIRC, the lifiters are solid, not hydraulic. The use a disc on top of the lifter to adjust the clearance; multiple sizes are available through Nissan to adjust the clearance, if necessary, but I've never seen or heard of a VQ engine that required valve adjustment. Also, it will require the correct tools and it's around 18 hours of labor, book time. I doubt that the high-mileage oil did anything, but if you've been using synthetic regularly to maintain your vehicle, I, personally, would stick with it. Whether you race or not is really not a sole determining factor of using synthetic. Synthetic is primarily beneficial to engines that see extended maintenance intervals or those that see severe conditions, like heavy towing or extremes of temperature. Most synthetics are good for 10,000 miles or more (I use synthetic in my 06 Pathy, but still stick with the 7500 mi. intervals per Nissan). I also prefer to stick with a genuine Nissan oil filter. One thing you may want to do is test the oil pressure with a mechanical test gauge to make sure it's within specs. If that's good and your sure the noise isn't due to power valve screws that have fallen out, you might try some Marvel's Mystery Oil added to your oil. It's not something I usually recommend, but seem people swear by it and it won't hurt.