Change oil sign came on,but i have synthetic oil, how can i restart the oil gauge?

There is nothing wrong with your car, the color of the smoke is a sign of good combustion.

A leaking thermostat housing shouldn't cause false temp readings, but if it's leaking it should absolutely be replaced, however it's more likely that you just need a housing gasket, unless there is some non replaceable seal.

As for the sludge problem, regular oil and filter changes with a good quality oil should keep it from forming. However a flush won't hurt and you can do it yourself. Just follow the instructions on the bottle. You can change to synthetic oil at any time without any problem. It is a common misconception that you can't switch between regular and synthetic, this is untrue, you just can't mix the two.

Keep up on regular maintenance and you should have no sludge problems for many more miles.

The 3000-4000 mile oil chnage interval is for extreme driving as described in the user manual. If you are not driving in stop and go traffic,frequent engine shut off's and restarts or dusty conditions, short trips, you can adhere to the higher mileage interchange. Your vehicle should alert you when service is due.

If this is a new problem since the oil change the oil or filter are the cause. Filters do fail and is between the pump and the pressure sensor most times. So if there is a restriction in the filter the pressure will be low at idle. Replace the flter and see. There is a chance the sensor failed coincidently to the change too. A direct read of the pressure with a gauge will prove that. If you hear any ticking or tapping turn of the engine.

it depends on what type you are using if you are using conventional oil (regular oil) i would change it every 3000 miles if you are using synthetic then you can go over 6000 miles.of course the sooner the better for your engine.

You could try thicker oil or stop using synthetic or add some lucas oil treatment or change the oil. The sender could be bad too. A diag is in order because if there is no presure the engine will soon die.

The oil pressure sensor is bad,did you by chance change to synthetic oil recently,my jeep did the same right after changing to mobile1 synthetic