Geargrinder,
To avoid the risk of sounding like the snakeoil salesman Lost1 is referring to (and he is right about the "buyer beware" and "old fogie" mentalities that lurk out there)...I'll explain my background before answering your statement:
In 2005 I retired as a US Army senior warrant officer. I spent 25 years inmilitary maintenance (15 enlisted and 10 as a warrant officer). After retirement I went to work as a Maintenance Planner in a borax mine/refinery in the middle of the mojave desert. In my former life as a soldier, knowing and administering the Army Oil Analysis Program (AOAP) could mean the difference between life and death. In my current life as a maintenance and reliability planner, having a good lubrication plan can mean the difference between profit and the loss of million of dollars. I will say up front that I am not a tribologist (and I don't play one on TV), but I rely on the advice/service from them...and since many of them are trying to sell me something...I have to know BS from truth when talking with them.
Now (sorry that got so long) I need to reply to your statement about milspec, and the constant error I see popping up here about compatibility:
MILSPEC- Short for military specifications is a label put on almost (but not quite) everything the
military buys to attach it to a set of standards. When it comes to what the military
determines as supply class number 5 (which is titled "Petroleum, Oil and Lubricants")
there are many milspec ratings. So many in fact, I won't BS you and pretend to know
them all. What I will tell you though is that they (milspec) speak to performance
standards, and not to interchangeability with each other. There are in fact oils (in this
case) with the same milspec rating...that are not compatible with one another.

Compatibility- I'll yank myself off the soapbox here, before I bore you all to tears (if I haven't
already) and say read my post(s) to this thread a little up the page & you'll see
that compatibility among different brands...not mineral Vs. synthetics is of more
concern to your engine than mineral & synthetic compatibility. Finally, I want to
ensure I don't over emphasize this...incompatible oils are not going to "blow up"
in your engine, or caus

 

Thanks clutch for enlightening us on this good question, important subject. I knew this applied to oiladditives, wasn't sure about theinterchangeability of oilbrands.