aha, it's protocol 47, not 40 that PPTP uses.
But alas it's all there.
Routing and Remote Access
The Routing and Remote Access service provides multiprotocol LAN-to-LAN, LAN-to-WAN, VPN, and NAT routing services. Additionally, the Routing and Remote Access service also provides dial-up and VPN remote access services. Although Routing and Remote Access can use all the following protocols, the service typically uses only a subset of them. For example, if you configure a VPN gateway that lies behind a filtering router, you will probably use only one technology. If you use L2TP with IPsec, you must allow IPsec ESP (IP protocol 50), NAT-T (TCP on port 4500), and IPsec ISAKMP (TCP on port 500) through the router.
Note Although NAT-T and IPsec ISAKMP are required for L2TP, these ports are actually monitored by the Local Security Authority. For additional information about this, see the "References" section of this article.
System service name: RemoteAccess
Application protocol Protocol Ports
GRE (IP protocol 47) GRE n/a
IPsec AH (IP protocol 51) AH n/a
IPsec ESP (IP protocol 50) ESP n/a
L2TP UDP 1701
PPTP TCP 1723
Computer/Network Technician
CCNA