I just bought a used 1997 Silverado, V-8 5.7 Liter.

They seller informed me the tranny fluid was changed with the filter about 10,000 miles ago. He said he used Amsoil.

I heard that when using Amsoil the fluid level does not heat up much therefore the cold /hot markings on the tranny dip stick allow you to fill the fluid up to the bottom of Hot Markings.

Is this true / any thoughts?

If I need to add some - The hot area of the dipstick has hashmarks. How much fluid would it take to fill up the hashmarks. This would be a reference point for me.

I haven't heard that but I also don't know anybody that has used amsoil. I would drive it and get the fluid up to temp. Then check the fluid level as long as it in the hash marks for hot it good it does not have to be at the top of them just in the hash marks. Also the fluid has to heat up for the temp sensor to know it has warmed up otherwise the tranny will run as if its cold all the time and might set a code. I think the fluid heats up just like normal fluid it just does not expand like normal fluid.

Amsoil, or any other transmission fluid additive, is so low in % volume of the total amount of fluid it will not dramatically affect the thermal expansion factor.

Thats not true, you fill the trans the same way as with any other ATF. AMSOIL 100% Synthetic ATF has excellant hot and cold temperature performance and helps you transmisson last longer.

I have AMSOIL in my 2012 Sierra AWD Seirra Denali and in my 2009 Cadillac AWD CTS, engine, trans, transfer case and both axles.

AMSOIL is not a additive, its a 100% synthetic automatic transmission fluid.