1998 BMW 535i engine misfire

i had the plugs and leaky rocker gaskets changed on my 535i (m62 engine). a week or so later after a steady drive it began running rough and got worse as i headed home. now it struggles to tick over and is misfiring badly. i checked all spark plugs and they are all fouled to some extent but numbers 7 and 8 were covered in oil. 5 and 6 were slightly oily. the ht on cylinder 8 was coated in oil all the way up almost to the coil. there is a bluey grey smoke on start up which then goes to white (or steam). when it first starts it doesnt misfire for about 5 seconds. any ideas on a possible cause would be appreciated. thanks