Halfrauds Semi Synth 10W-40 Motorcycle oil is a general favourite on the forum. Good price and it's 5 litres instead of the usual 4 litre sized containers that most other manufacturers sell. The oil is rebranded Comma oil and it's decent stuff. You can get Filtrex filters on eBay for £6, they make loads of OEM filters so they're good quality. Alternatively use Hi-Flo filters, they are fairly cheap too and people on here and on the DR-Z forums I frequent are all happy with them.
As kodkod has mentioned, forum members get some nice discounts at Opie Oils. Any 10W-40 semi or fully synthetic motorcycle oil from them should be fine for the Stroms. They sell good oils all round so brand isn't much of an issue unless you have specific preferences.