I've started to notice a grumbling/grinding noise when I'm parked up and turning the steering from lock to lock. Feels fine when driving and it doesn't wander all over the road so I was wondering if they just need the oil/grease changing.

I've had a quick look underneath and they don't appear to be pitted, no oil leaks either.

Is it as simple as it looks to do? Just drain the old oil and refill?
Which oil and how much should I put in?

Cheers...
(Ex-utility, 2005 Defender 110, Hardtop)

jack both wheel up drop drag link and feel for play and roughness in turning locl to lock if bearings are worn they need replacing asap

I wouldn't have thought it was the swivel making a noise. Unless the bottom bearing is totally shot but unlikely.
It could possibly be a track rod end.
Can you get someone to turn the wheel lock to lock while you 'feel' around for the noise? hold each track rod end and see if you can feel it creaking.

Since you asked about checking swivels, jack the front end up and put on axle stands, give the wheels a tug top to bottom to check for any play while it's up there. If there's play it's either wheel bearing or swivel. Take the wheel off (might need to lower it back down to loosen nuts if you don't have an impact gun). You'll see a plug with a square top on the front of each swivel. Remove these and fill up with EP90 or one shot grease, or a mix of the two which is what I do. Fill till level with the hole and your done.

... ah beat me to it James

Thanks gents, will have a look this weekend...