97 Chevy S-10 Oil Pressure Problem

My mother owns a 97 S10 4 cyl truck and recently replaced the water pump and thermostat. Now theres a problem with the oil pressure it seems. It always registers low unless its very cold outside. After driving for only an hour or so the engine begins to knock a bit and the check engine light comes on when sitting idle. It seems to run ok but she wont drive it very far for fear that it may blow up on her. It seems to have good power.[/B][/quote]