Can my ISP see which VPN protocol I am using?
I know that my ISP can detect when I am using a VPN, but can it see which protocol I am using?
Yes, probably. Most VPN protocols are not designed to hide the fact that they're VPN protocols, nor what kind of protocol they are. See for instance this paper which details fingerprinting OpenVPN.
If you run all your traffic through a VPN, the fact that you're using a VPN is quite easily visible - as all traffic will be destined for a single destination - which is quite unlike normal usage patterns.
If you're interested in hiding the fact that you're using a VPN, and what VPN, you should probably look into what's used in totalitarian states, such as China, which routinely blocks all attempts at VPNs out of China. Tor is one such system.