Looking at a list of loco-hauled services and RTT, that one's back to being a 165, at least for now.I think that makes the 08:26 TuWTh Gerrards Cross to Marylebone the only regularly scheduled loco-hauled service which Oyster cards are valid on, as it calls at West Ruislip at 08:36.
Using contactless is still an option for services with calls at or south of Princes Risborough (or, separately, between Cardiff Central and Newport if wanting to use any loco service using PAYG).