1991 Isuzu Trooper heating, but not overheating.

I have a 91 trooper 4ze1, my dad took care of this car by the book. When I got it. It was running great, I changed the oil, to 15w-40 usually we used 10w-30, and ever since then, the temperauter goes up way up to the H on the gauge, not to the H but almost there, after a while it will cool down sometimes, other times it will stay almost at the H. The pressure for the oil is almost always 75 its supposed to be at 30-55. Could this have an affect on the temperature of the engine? Im going to go back to 10W-30 because it was running fine with that. Plus the heater doesnt work so the engine doesnt get that cooling effect, me and some friends are working on trying to fix that.