Is Amsoil 10/40 ok to put in an engine with 100,000 miles?

 

I'd ask why you want to use the 10w40. Your bearing clearances really should not have changed much so unless you track your car, a 30 grade is probably better.

If you are looking for an oil with higher amounts of ZDDP than allowed by the API in current API SN spec oils, I'd consider this one:
AMSOIL Z-ROD 10w30 Synthetic Motor Oil (Product Code ZRTQT)
(zinc – 1440 ppm, phosphorus 1320 ppm)

Note that this oil doesn't meet GM Standard 4718M which was the spec for the C5 (superseded by dexos1), but it is very similar to the AMSOIL racing 10w30 I use but it has enough detergent and dispersant for about 3-5k oil change intervals.

A 10w40 is probably okay to use in your Vette, but you'll pay a viscosity drag penalty which will hurt net hp and mpg. That said, I used this in my car at the track for several years prior to AMSOIL coming out with a true racing 10w30 (and note the internal of my engine are stock):
AMSOIL SAE Synthetic Premium Protection 10w40 (Product Code AMOQT)
(zinc – 1378 ppm, phosphorus 1265 ppm)

Again noting that this oil doesn't meet GM's recommendations and in my case, I used it at the track where my oil temps are about 40-50F higher than you should see on the street. If you do use a 10w40, be very easy on the engine until the oil is warm. My personal thumbrule with my car is not to get into the throttle hard or use more than 3000 rpm until my oil is over 150F.

Finally, if you do opt for AMSOIL, more than happy to get it for you at my cost, about 25% below retail, via the AMSOIL Preferred Customer Program. Drop me a PM if interested.