Jackson's coat doesn't ever get too greasy, really. But I do notice if he hasn't had a bath... he needs bathed weekly to really stay 'fresh'. When it gets longer though, it is harder to manage and keep clean looking which is why I keep him short. Yorkie owners who keep their dogs in long hair definitely spend time grooming every single day - it's really a necessity for a long coat. I don't have the patience for it and neither does Jackson. He's very easy... I rarely brush him and just get him groomed professionally every 10 weeks and bathe him once a week.
His hair is pretty soft though even when longer. Yorkies pretty much have three coat types, though not all are standard. But you've got your cottony, silky or modified coats. And of course, their hair is more like human hair, so just like us... when not washed for a while, it's going to get greasier. However, washing TOO much can also destroy coat. A lot of Yorkie owners have great luck with Chris Christensen products as well as Minksheen (which I use) and I follow up with BioGroom Silk Creme conditioner.
Jackson is modified. He's not really 'silky' and doesn't have silky straight hair but he's not cottony, either. When his gets longer, it gets kind of curly-ish. Some Yorkies just have naturally fine and thinner hair, which WILL be more "greasy" looking and from what I've seen, there's not much you can do about it.
Also, if Ada has never had a proper haircut in her life, it may just need to be shaved down and started over... I remember Jackson's puppy hair being a pain in the butt. I'd brush it, it'd look nice for 10 minutes, and then it would be all messed up again. I was so glad to get rid of his puppy coat.
From some Silky's I've seen online, there's quite a few who have that 'greasy' look, though I like the ones who DON'T have that look. lol.
Silky Terrier:
This was the most annoying puppy hair that was always tangled.