Think for oil you should be looking for something a bit thicker, 10w50-60'ish. £400 sounds cheap for a 90000 mile service to me, from a dealer. I was expecting mine to be about £700, but never found out as I sold the car[]. Def try a specialist. Seem more flexible than dealers, ie if you just want an oil change, or intermediate service, or a cambelt etc that's what you get, not you are due a 30000 mile service, therefore you WILL get a 30000 mile service or your car will blow up and all the wheels fall off!![].
Had to give my green filter quite a trim, but not had a problem with it. Is it sitting right in the airbox, mine was quite a job to get in.
P.S when I traded mine in the dealer didn't seem bothered it didn't have a subaru history (it was specialist), more that it had just been serviced by someone. Surprised me a bit tbh.