[steve s]

Ford's original recommendation was 20 wt in the winter and 40 wt in the summer. So, your should be fine with 15W-40, plus you'll be prepared if for some reason you should want to start it up in cold weather.

Oil is one of those topics of endless discussion, debate, and pontification (e.g., thermostats, antifreeze, filters, etc. ). My impression is that a plurality of people (if not a majority) just use 10W-30 and call it good.

One of the legends (some would say, myths; others would say, truths) is that you shouldn't use detergent oil in a motor that may have been run with non-detergent for a long time because it will break loose chunks of accumulated sludge. I believe that this has now been long debunked.

Lubrication experts will point out that STP is not what they would classify as a lubricant, and it is well to remember that adding it, or any additive, necessarily dilutes the concentration of the real lubricants and other professionally vetted oil components. But, it's a testament to the Model A's durability that this, and many other dubious practices, can be followed with no apparent harm.

Steve