hi i have audi a6 petrol 18 t on 99 plate .was thinkin of gettin it serviced,was wonderin wat oil to use .i ve always got the castrol edge fully sinthetic.5w/30 as reccomended by castrol .was £35 for 4 litres but now in halfords £45 a bit steep i thought but the sales person in there said i cud use part sinthetic 10/40 and also when i phoned the main dealer they said they sold the 10/40 for the service,am confused?? i service the car yearlyand was wonderin wat to use.....also halfords sell spark plugs 4 for £20 borch while audi s are £11.50 each wat would u recommend ...many thank s in advance