Any correctly weighted oil will do you (10w50) Shell, Mobile etc all do it. Any decent motor factors should be able to track a filter down for you.
This is probably a question only John501 can answer.....I have a large 25L container full of fully synthetic Miller 10w60. Will using this void the warranty, should I need to claim on a new engine and the oil is tested?
I don't believe it should as the 60 designation indicates that the oil is able to withstand higher temperatures whilst maintaining it's viscocity.