I would assume the engine has hydralic lifters. A heavy oil may help with some noise if it is lifters, but won't cure the problem. I know some will hate me for the suggestion, but you could try some STP oil treatment to help thicken up the oil. If you change the oil you might try a straight weight oil (30W), but that won't help much in cold weather.
Does the engine burn much oil, or burn blue out the exhaust?
Also get a cheap diagnostic stethiscope. It may help pinpoint the area that the noise is coming from.
If you have a oil pressure guage that displays actual PSI, does the oil pressure drop off real low after it warms up? It may indicate a rod bearing problem. What are the oil PSI's when cold idle, warm idle, cold when driving, warm when driving?
