Does 10w-40 oil can damage my engine? Instruction manual says "only 5w-30 oil can be used in LT1" My father recently bought 40 quarts of Purpule Royal 10w-40 but I am not sure if I can use it in my Z28.

 

You can definitely run that in your engine, in fact I may be switching to Royal Purple 10w40 next year myself.

GM recommends 5w30 for fuel economy, but you'll probably find better engine wear with an oil like Royal Purple's 10w40, which is formulated on the lower end of the 40wt scale.

Keep in mind that as an engine ages, it will show better engine wear numbers with a slightly thicker oil than it did when it was new. So an engine that shows it's best wear numbers (in oil analysis) with a 10w30 when new, will then need a slightly thicker oil once the miles go up.

Also keep in mind that there is a big gap between the viscosities of some 10w30 oils. An oil like Mobil 1 10w30 is on the lower end of the 30 scale, at 9.8cst at 100c, while an oil like Amsoil 10w30 is on the higher end, at 12.0cst at 100c. The range for 30wt is 9.3 to 12.5. Then 40wt is from 12.5 to 16.3. RP 10w40 comes in at 13.8cst at 100c.

So in other words after all this long winded explanation, that oil is totally safe to run, and I'd choose it over Mobil 1 10w30 in a heartbeat.