1990 nissan maxima has no heat after changing thermostat and water pump AC works

Question edited for clarity.

No to both. The water pump is chain driven and takes water from one side and pumps it out under pressure from the other. The thermostat is fully enclosed in a housing with no access to oil.

Oil in Coolant only comes from a failed head gasket where the oil and water channels are in close proximity and under combustion pressure of hundreds of pounds.

Water pumps are designed to pump water through the engine, radiator, and heating system. Water pumps will not cause oil to enter the cooling system.Oct 4, 2016

Q: Can a bad water pump cause water in the oil - YourMechanic

Water Pump
https://shop.advanceautoparts.com/find/nissan-maxima-water-pump

Thermostat Housing
https://www.google.com/search?q=nissan+maxima.+thermostat+housing

..

Check the hoses to the inside heater core for kinks. Check heater (not radiator) core for being plugged. Check heater valve to see if it lets coolant through when turned on.

Could be the radiator fan. Running the heater adds a second source of getting rid of heat in the coolant. Is the radiator fan running when it is hot ? Are you sure the cooling system is full ?

sounds like the water temp sensor is not working you could hot wire the fan by unpluging it and runnig ahot wire and ground to it

check your thermostat and possibly your cooling fans and temp sensor for the fans

Make sure there is no debris in front of your radiator or ac condenser

Check radiator for flow. Blockage. Should have no cold area when car is hot

it depends on why the heat is not working
it might be the thermostat
heater core
heater blower or heater control panel

its possible that u have a bad water pump.

not at all times can thermostat contol your cooling system, check for water leaks, water pump, radiator fans(check for speed) if all your cooling systems are good , replace your cylinder head gasket,

might be the heater core.