From reading the company literature on Primrose, the best I can tell is 409 and 405 appear to be essentially the same with one exception; 409 contains additional chemicals to prevent Gelling of diesel fuel in cold weather. There are no warnings not to use the 409 product in the summer, so you should be OK.
405 is mixed at 1 ounce per 15 gallons (12 ounce bottle per 187 gallons per bottle instructions)
409 is mixed at 1 ounce per 8 gallons (12 ounces bottle per 100 gallons)
I would say your wasting a little bit of money by using the 409 year around. Why not just order some 405 for the warmer months? It's almost a 2 to 1 difference in mixture ratio.
I have ordered both 409 and 405. Will use 405 in the spring, summer and fall. Will use 409 only in winter when I go camping/motorcyle riding in SoCal desert.
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