Q: Low idle, lots of smoke from the exhaust, feels like no power. Mechanic looked at it and said to change my oil grade?

Hi there. From the description you have provided, there is a combination of things happening with your vehicle. If the exhaust smoke is oil smoke, increasing the oil viscosity could help reduce the oil consumption. It could be, the piston oil control rings have failed/ are worn if you see the oil smoke upon acceleration. It could also be the valve stem seals have failed if, you see the oil smoke upon deceleration or extended idle. This smoke should appear white with a bluish tint. Black smoke and it could be excessive fuel consumption. The low idle and lack of power could be the exhaust system, ie; catalytic converter partially restricted from the oil smoke. Before replacing the catalytic converter, the smoking issue needs to be resolved. I recommend having your vehicle’s smoking exhaust be inspected and diagnosed by a certified technician, such as one from YourMechanic.