I'm not altogether sure how relevent it is to the Hemi tick problem but I found some of the comments in this Porsche thread very enlightening...
https://www.pistonheads.com/gassing/topic.asp?t=1392764
In short, a Porsche owner who, because of working overseas, let his car sit for three months, always experienced a lot of tappet noise on first start-up. The clatter took twenty minutes to clear.
Some wise bod suggested that valves pressed in the open position on engine shut-down would over time see oil bleed from inside the hydraulic tappet (via the NRV) to leave behind an air gap. I found the guy's description of how the air in the hydraulic tappet might 'fight' against the inflow of new (cold) oil quite convincing.
This is probably close to being a dead thread now but I'm going to say something that I think might be important...
Some of you know I'm a ex-oil formulator, responsible for inflicting billions of litres of engine oil on an unsuspecting world! My job was to get oils to meet various specifications in the most cost-effective manner. Typically this involves fixing the oil's resistance to oxidation, deposit formation tendencies and wear performance to pass various engine & rig tests.
However at no point in my career did I ever consider how oil might impact on the lubrication and long-term operation of either hydraulic tappets or the simpler bucket & shim variety. In fact it's only in the last week or so that I've figured out what they do and how they work! And now I understand them, I see a lot of potential for problems.
For example, oils are basically meant to MOVE. However, if I've understood things properly, once a hydraulic tappet piston is full of oil, the oil basically stays there forever with no obvious means of exit. Changing your oil might have zero impact on the oil trapped inside the hydraulic tappet. Over time there's an obvious risk that this trapped oil will degrade and insoluble gunk will plate out and cause lifter stickage problems. From what I've seen on YouTube, Miata engines seem to be particularly sensitive to this problem.
The thing is, as the world (and particularly the US) has moved in the direction of ever thinner, more stable, hydrocracked oils, this has been accompanied by a marked drop in the fundamental solvency of oils. These oils may produce far less gunk but their ability to keep any gunk formed in solution is far less. Thinner also usually means more volatile (higher Noack) so you have a greater potential for 'burnt oil' deposits to get caught up in your engine oil. Will these be kept in the oil by Ashless Dispersants in the same way as 'normal' oxidation insolubles formed in-situ in the oil? I suspect not and it may be that long-term problems don't manifest themselves in the traditional 'problem areas' like the ring pack but in the more sensitive (and often overlooked) areas such as tappets.
I am beginning to think that the oil world might be a better place if the very cheapest oils on the market (all Group II US 5W30 GF-5 oils for example) were all formulated to contain 10% polyol ester? It would put back the 'missing' solvency and have a significant impact on reducing oil Noack. Both things might improve the long-term operation of engines.
