In Short - Yes
Additives can be mixed without harm, but you should take into consideration what each product is meant to do as to not overdose on certain items. The major categories of diesel fuel additives include engine performance, fuel lubricity, fuel stability, and contaminant control additives. An example of adding too much of one item could be adding too much cetane improver. Too much of anything can result in less performance than the recommended dose.
We see a lot of people trying to achieve different goals and mix different additives. A great example and probably the most popular mix we get asked about is XL and Boost! products being mixed at full strength. Due to what these additives are designed to do, one as an extreme lubricant and the other as a cetane improver, they complement each other well. But you would not need to mix XL and XPD as they both have high lubrication properties, so one would cut back on the recommended dosage rate if they were mixed.
You may also just get the same benefits of mixing XL and Boost! as you would with just running Summer+ by itself.
