I have a 95 Z28 5.7L I would like to use a high mileage oil my car it calls for 5W30 is there any problems with using a heavier weight like 10w30 or 10w40 in synthetic,and which brand is the best Castrol,Valoline Maxlife,or Quaker State or Pennzoil. This is a summer car only I store it in the winters here in Maine.

 

First some factual information, then I'll give you some opinion.

For summer driving, 10w30 is not thicker than 5w30. The respective 10 and 5 in those numbers reffer to the oil's ability to flow at 0*C. The 30 in both of those numbers represents the oil's viscosity at 100*C.

Many people use 0w40, 10w40, 15w40 oils in their LT1 and I've never heard of a lubrication related failure caused by using a *w40 motor oil.

There are lots of theories out there saying that higher viscosity protects your engine better, and equally as many that will tell you that lower viscosity with better flow will protect your engine better.

GM says to use 5w30 for all temp ranges and will allow 10w30 if it doesn't get extremely cold where you will be driving. If they assembled your motor, I'd stick with their advice. If you're running a rebuilt motor, I'd check with your engine builder.

As for brand, I can't say that any one is the best and even if I could, they change their formulas about as often as I change my oil.... If you use a quality oil of the proper weight form any of the oil companies you listed as well as several others, you are unlikely to suffer a failure that was caused by your oil.