Hello I'm new to this site. I recently bought a 1988 Chevy Silverado w/ 5.7 350 3 speed. Engine has 120,000 miles and was taken care of (driven by older couple). Oil was changed regularly and looks clean on dip stick. I have only had it for couple weeks. Recently drove about 20 miles and when came to stop sign with clutch pushed in got check engine light. Looked at gauges and oil pressure was at about 10, when driving down road light goes away when oil pressure rises to about 15-20. Engine does not get overheated, not knocking, idles well. Should I try using a higher weight oil like 10w-40 or go right to changing oil pump. I hope its not bearings but how would I be able to tell? It gets cold here in iowa and also wnder if heavier oil couold mean harder starts this winter. I forgot to mention that start up oil pressure is around 30. Stays that way until temp goes up. Thanks.