1994 Other Jeep Models only works on low speed

Heater problem
1994 Other Jeep Models 6 cyl Two Wheel Drive Automatic 130 k miles

my heater only works on low speed, have u ever heard of something so silly? What can possibly be wrong beside it is nuts. Thanks alot. Also now my husband just changed the oil and filter and now the oil pressure gas wont work, any suggestions and can I drive this vehicle with the gauge not working, I know their is new oil in it. Please advice.