I know a lot of contractors recommend FLAT, but, I hate a flat finish for exteriors. Any dirt in the air tends to find its' way onto your siding and really collects on a flat finish. Satin is best IMO for most substrates.
I prefer flat for the field areas like siding, stucco (if painted, ouch, would never put a sheen on stucco). BM's low luster is also nice, for non-stucco areas. I like a semi, or lower, on exterior trim. And gloss for the doors. If you have a lot of field area, a sheen can tend to look gaudy. While exterior flats are far more resilient than interior, they will attract dirt quicker than sheen enamels, but that's why they sell house wash chemicals and equipment. It's not extra work really, as all paints will benefit from a regular cleaning. Along with visual inspection and spot repair/touch-up of exterior painted surfaces, a regular cleaning is smart maintenance.
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