The Oil Pressure Gauge reads above 40 and is normal for a while, but after about 20 minutes makes it'* way down to a precariously low reading of 20 and sometimes below. Also there is a rattle in the engine compartment at startup. Do you know what the problem could be?

Oil pressure is fine now, but there is still a rattle at startup. Also after adding some oil there was a burning smell.

Yeah the oil was dark and needs a change, just added more until I can change it on saturday. I'll try to get a better description of the sound a little later.