Oil filters for AEC engines are readily available from commercial vehicle parts suppliers. Ask for FLEETGUARD LF 4017 or FRAM PH 2829 or equivalent, price about £5+VAT
I am very grateful to Mark Allin for letting me post details of his research into the availability of filters for Routemasters.
All the filters used in Routemasters, including all the engine types commonly in use are available from Filtration Control Limited who supply most of the major national bus companies.
Filtration Control Ltd, Filtration House, Units B/C Longman Court, Sketty Close, Brackmills Northamptonshire NN4 7PL phone 01604 707750 fax 01604 707751
Details of the filters required and their part numbers are available below. Email firstname.lastname@example.org.