2003 Chevy S-10 Oil Recommendation - High-Mileage 4-Banger

Bought this truck about 2500 miles ago. Engine seems pretty solid, considering that this was a fleet vehicle for a number of years. I have a couple of concerns regarding the engine oil, though.

At cold start-up, there is a brief knocking that appears to be coming from the bottom end. It's very brief, and disappears after a couple revs. When cold, oil pressure reads a little over 60 on the panel gauge, and drops down to 20 when fully warmed up @900 RPM. I'm thinking rod bearings might be a little on the worn side, although probably not a big issue at this time.

I'd like to insure that I get maximum oil protection at startup, as well as hedge my bets during the hot summer months with the A/C running constantly (those days are right around the corner). The majority of time will be spent on city roads and interstates.

I'm thinking about going with Mobil-1 high mileage 10W-30. I've heard good reports about synthetic oil in older engines, and currently have no oil leaks to speak of. What's your take - you reckon the benefit is worth the extra $$$?

GM recommends 5W-30 for this engine, which is what I've been running all winter. I figure the 10W-30 might help with the cold starts, all else being equal. Since this engine has probably been run pretty hard for most of it's life, do you think it's prudent to consider a 10W-40 for high-temp protection as well?

One thing I've learned over the years is that good oil and frequent changes will keep a lot of boogymen away. Appreciate any information about this particular engine.