Notes on the Pentium's microcode circuitry

Can I add my own instruction set extensions to the original x86 isa as implemented by the 8086 without permission from Intel and / or AMD as long as I'm not copying any x86 instruction set extensions?

I don't know the legal details here but I think you can do whatever you want as long as you're not violating any patents (good luck). Also, Intel claims a copyright on the mnemonics for 8080 and 8086 assembly language. Microcode is also protected by copyright.

Any patents on the 8086 have long expired, and so have the ones from the last century. As Ken says, the microcode is copyrighted but you don't need to use that to make a compatible version.