Do not use the wrong oil.
I am certain that costco do 20litres of 5w30 oil for about £25. It's chevron brand. I have been tempted to use this in my Audi, but have not as from the blurb in costco it makes no mention of VW507 spec that my Audi asks for. For you purposes it should be perfect.
My general rule for oil is that it's ok to use a thinner oil, but not a thicker one. For example my Volvo asks for 10w40. I have happily used 0w40 fully synthetic and 5w30 with no ill effects (in fact if your piston rings are ok, the only side effect should be better MPG).