Preheat a grill to medium-high heat.
Coat the corn evenly with the oil. Grill the corn, turning so that it cooks evenly and grill marks form, 10 to 12 minutes.
Mix together the mayonnaise, Sriracha, lime juice and chili powder until smooth.
Coat the corn with the mayonnaise mixture. Sprinkle with the chopped cilantro, if using, and garnish with more chili powder for extra heat.