I agree with the HDEO 15W40 comments, flat tappet cam being one reason.
You might also consider that the head on this engine was designed when leaded gasoline was available everywhere.
Lead content in the gas helped lubricate the relatively soft metal in the exhaust valve seats.
Today's unleaded may (or may not) result in accelerated exhaust valve seat wear in the heads.
There are lead substitutes available, and the effectiveness of these lead substitutes is debatable.
Cut and paste from Yahoo autos.......
"Valve Recession
A condition known as "valve recession" can allow the valves to recede or sink into the head because of excessive seat wear. This causes the valve lash to be lost which allows the valves to leak and burn. It occurs primarily in older engines (mostly those built prior to 1975) that were not designed to run on unleaded gasoline. When leaded gasoline was still around, lead acted like a lubricant to reduce valve seat wear. But when lead was eliminated, it meant engines had to be made with harder seats. These older engines didn't have hard seats, so many began to experience valve wear problems when switched to unleaded fuel. If you're driving an antique or classic car, therefore, you should either use some type of lead substitute fuel additive to protect the valves or have the seats replaced with hard seats when the engine is overhauled."
At 36K miles I assume that the engine has never been opened.
Should a valve job ever be necessary; hardened exhaust valve seats could be installed at that time.
My dad had a Ford van similar to to this back in the 60's.
As I remember it had some some tendency to overheat.
Rickey.