PREP: Line a large sheet pan with parchment paper and set aside.
COMBINE: In a very large bowl, combine the Corn Chex, miniature pretzel twists, Rice Chex, Honey Nut Cheerios, and 1 cup Valentine's M&M's. Stir to combine.
WHITECHOCOLATE: In a medium-sized microwave-safe bowl, combine 3 cups of white chocolate chips and 1 and 1/2 teaspoons vegetable oil. Microwave for 30 seconds, stir for 15 seconds and then microwave another 15 seconds. Stir until melted and smooth. If needed, return to the microwave for a little longer. (Bursts of 15 seconds stirring for 10 seconds in between each burst). Pour the melted and smooth white chocolate over the snack mix.
COMBINE: Working quickly, gently stir until all ingredients are coated in white chocolate and then quickly transfer to the prepared baking sheet and spread into one even layer. Immediately add the remaining Valentine's M&Ms on top and optional sprinkles. OPTIONAL STEP: Melt the last 1/4 cup white chocolate chips in the microwave and then transfer them to a small plastic bag. Cut the tip off the bag and pipe stripes across the white chocolate snack mix.
FIRM UP: Allow white chocolate to firm up and harden at room temperature or place in the fridge until firm, about 10-20 minutes. Break into pieces and serve.