Another thing on these engines you might want to check out that I run into alot down here is at about 150k the timing chain guides take a crap so you may want to check for pieces of plastic in the oil when you do an oil change. I've done at least twenty in the last three months. If you do run into it drop the pan and clean the pickup tube out. All that plastic goes somewhere and it's usually the pickup.
Thats not a problem for our cars and Pulling the oil pan on his car is a PITA.
Had a 2007 and a 2004 Town Car in the shop today. Did rear axle shafts on the 2004, it had 434K on it. Original Chains and guides. Mobil 1 Extended performance oil and extended performance oil filter done every 15K.
2007 just did control arm bushings, shocks and new front struts. 377K on it. Same oil and filter.
The chains and guides on the cars i service will make it to 500K.
I've only replaced the chains and guides on one E350 5.4 van with 230K, that was still considered low mileage but when we opened the motor up we found the motor was not maintained.
Don't touch spark plugs in these things until 300K usually.
The origional engine out of my 2000 Crown Vic was still on its stock guides and chains with the 190K on it when i took the motor out and shoved it into another town car that DID blow the guides and it bent some valves. But that town car had 513K when it started getting noisy and the driver buried the go pedal.
edit: i just realized you might be working on Civilian cars. The cars i maintain are all fleet cars racking up hwy miles and run 24/7. One driver gets out, another jumps in. The only time the cars get shut down are for oil changes. So the mileage we see out of the guides is going to be much different from what civialian car sees.