Which type of spark should i use in a 1991 ford aerostar with high mileage? Regular plugs? Dual platinum? Or iridium?

I see no reason to change the heat range of your plugs if you are not using any oil. In any case, you could always go up one heat range in standard plugs bearing in mind that the other 2 aren't cheap.

Autolite Iridium They last a very long time and with the regular rebates they are affordable. Also as you may know the replacement of the spark plugs is very intense and time consuming. So save your time and money. Buy the Iridium plugs, the ones for regular driving, the race plugs do not last as long.

Auto lite is the best for American cars. Japanese NGK, EURO Bosch....(note: threading varies, albeit every so slightly, between these brands) STAY AWAY from PLATINUM for any model vehicle, unless you plan on changing them frequently (due to racing type driving) on a VW they cause bug to run hotter, and plugs don't last!