The manufacturer only recommends WD40 for chain cleaning, not lubrication. As above, it'll remove the grease and then disappear.
Chain lube can be had dirt cheap in a spray can and is the way to go. The best thing you can do is to use small amounts regularly. If you don't have a centre stand like me, get a £25 little stand which props the back wheel up (not a big paddock one) so you can spin, spray, spin, repeat. Getgeared sell these. As for lubes, don't bother with "racing" shite or any of that crap. Just get the £5 a tin stuff, make sure your chain is clean so you don't get all the grinding crap through winter and spray it on thinly. Do that every week - it'll take you 10 mins tops, your chain will last longer and when you get to that service, they'll hopefully not have to replace the chain as it has been well cared for. Also, use the time to check the play and adjust the chain if required as per the manual - another important bit of maintenance for the following reasons:
1) making sure it stays on the sprockets (i've seen them come off whilst people were riding - scary)
2) ensuring rear suspension travel isn't limited
3) ensuring proper gear changes
4) making the chain last.
A craply maintained chain can last only 3000 miles, especially in winter. A properly maintained one will last years.