Sauté sliced mushrooms and chopped shallots in 2 tablespoons hot olive oil in a large skillet over medium-high heat 7 to 8 minutes or until tender. Remove from heat.
Whisk together remaining 4 tablespoons oil, juice, mayonnaise, and seasoning in a large bowl, blending well. Add cooked pasta, mushroom mixture, olives, and pimiento, tossing to coat. Cover and chill 8 hours, if desired.