2000 Ford Focus help!

Alright it's a 2000 Ford Focus ZTS with 235k on it. I got the oil changed the other day and afterwards the heat wouldn't blow hot until the engine temps were up around the middle on the gauge which is normal running temp for it. This is unusual for the car because the heat normally works as soon as the guage comes off cold. It seems the thermostat is opening up more often also. I then noticed white smoke blowing out the exhaust this morning. I normally use 10w30 on the car but the shop put in 5w30 this time. I checked the oil and it doesn't appear to be milky. It has been cold here in Dallas lately. Do I have a head gasket issue, a heater core issue, or is the car reacting to the lower weight oil? What else could be the issue?

Thanks

Once the oil is hot, they are the same.

Sounds like leaking head gasket.

Your heater issue may be to low coolant level.

I checked the coolant level in the reservoir and it isn't low but I am gonna keep an eye on it and also keep watching the oil.

Keep an eye on the coolant level. May be a bad thermostat or a leaking thermostat housing(they are known to crack.

Also the temp gauge on the Zetec does not actually check the coolant temperature. Sensor fits in a pocket in the head that does not have any connection to the actual cooling system.