10w-30 or 10w-40 for my car?

i have a 87 chevy caprice with 83,200 miles on it what kind of oil should i use? im in chicago now its winter well not really today we hit 62f so weather is always changing thanks to global warming....:( but anyways should i use synthetic oil or regular? im gonna go with mobil oil on my next change...and my car takes only 5 quarts or 6? its a 305 5.0 v8...my engine is making some noise so i wanna use some good oil on my next change hopefully it takes it away i was thinkin of using mobil regular 10w-30 oil and adding a hy-per lube oil additive maybe it can help my engine out a bit but the car rides super nice and shifts well my mechanic said the noise is commin from the engine though...