I use Pellgunoil on my HW40's, i put one drop on the piston and leave it standing upright over night so it runs down to the seal, i do this every 3 or 4 tins of pellets or every 3-4 months if it's not being used much, a smear on the breach seal is also a good idea.
have a look at Dowlings (link below) as far as i know they still stock it, no offence to our sponsor but i couldn't find any on thier site.
http://www.dowlings.uk.com/acatalog/..._Gun_Care.html
Hope this helps
Jason