I'll have to look tomorrow. I dropped it off at a shop.

I will try that tomorrow. Thanks.

Good to know. Thank you.

Well, got the truck back. I'm not sure what led to what, but it appears by crank position sensor is bad. But, it's replaced and the truck still runs rough at idle and feels a bit weak. The mechanic put a compression gauge on each cylinder and found all were at 100 psi except for cylinder 3 and cylinder 5. They came out 25 and 55. So, it appears that is the problem, but I don't know why bad compression would literally happen overnight....?
And what's next? Move to a thicker oil and see if helps, or is this engine done?

Do a wet test as well. If pressure comes up significantly, that's rings. If it does not, its likely valves.

Also, keep in mind, 100 is the MINIMUM spec for compression, so, the engine is marginal even without taking into consideration two dead cylinders.

Leak down test is next for sure. If it turns out to be the rings, can I just put new rings in or is it new engine time? What about trying some RESTORE in it?

Also, since all the bad cylinders are on one side, could this be a bad head gasket letting air past?

Could be rings, head gasket, or even valves. You aren't going to find a 'fix in a bottle'....... maybe if you had been using it for the last couple years, it would help, but, I suspect it's too late now. (if rings are even the problem.) You might be able to get away with re-ring job. (assuming that is actually the problem.) but, need to check cylinder bores for out of round, and taper. If they are too far out of spec, just doing rings won't help.

So here's an update on the situation. The wet compression test didn't change the reading much, so we are thinking it's valves. Took the valve cover off and there are no bad valve springs, pushrods all look good and move freely and with full travel. So it looks like the low compression is due to worn valves. My mechanic thinks I should just buy a used engine with lower miles and drop it in rather than tear this engine apart to do new valves. What would you do?
I think the engine is worth new valves. This engine has been pretty babied. I've run full synthetic oil and premium filters in it since day one. I did 3,000 mile intervals until last year when I decided to go to 5,000. I've never run it low on oil, never abused it, and I've never taken it over 3,000 rpms. It ran beautifully until all this started to happen a couple weeks ago. I've always heard that these 318 Magnums are near bullet-proof, so is there sense in dumping it and getting a newer used one?

Don't bother doing anything with the stock heads, just replace them. Get NEW castings. They are thicker, and less prone to cracking. (like the stockers do...) You can pick up a pair, ready to bolt on, for around 600 bucks. Or, if you are feeling froggy, you can even upgrade to the EQ heads....... Either way, just scrap the stock heads.

Poor engine never got broke in! Never run over 3k? Never worked? Never anything but synthetic? How many miles does it have?

I have pushed mine so hard and far that it should have blown many times if it weren't built so tough. Yeah, it's wimpy, but it just keeps on going. Have to tow and hit redline a few times on these work horses. Keeps them healthy. Who lives longer? A guy who lifts weights and runs once a week or a guy who never leaves the house and just sits on the couch?