Don't mix and match weights but switching from synthetic to petroleum (why would you?) you will not need to flush out your engine to clean out the petroleum oil. You'll notice you can buy petroleum, synthetic or a blend of the two at any auto parts shop. Stick with a quality synthetic (AmsOil, Royal Purple) in 5-20w and you'll be good.
2016 Premium Eco w/ PP - Roush Quarter Window Scoops, Side Scoops, Rear Spoiler; Modded GT Mesh Grilles; FP Strut Tower Brace; Eibach Pro-Kit Springs; Steeda IRS Bushings; BMR Billet Delrin/Spherical Vertical Links; Mishimoto Oil Separator; MAPerformance CAI; Mishimoto Intercooler/Aluminum Charge Pipes; Turbosmart Kompact Dual Port BOV; Turbosmart IWG75 Wastegate (10 PSI); Mishimoto Coolant Tank; JLT Engine Cover; Mishimoto Downpipe with modded MBRP XP Catbacks for TRUE 3" exhaust from turbo to tips; Drilled/Slotted Brake Rotors