I recently started experimenting with oils, opting to smear a magenta/purple oil wash on my Great unclean one (on the tentacles, stomach and guts) with rather good results, if I do say so myself:

So, yeah, oil washes are good, if you want to use them, but what if you don't? Well, have no fear, I still have some tips for you!
There's a little oil wash on the back here, but much of the purple/magenta is actually done only with thinned acrylics and some washes and contrasts. AND, it was surprisingly easy, as I basically just splattered it on semi-randomly and let it dry, not caring if it dried slightly blotchy (in this case that was just a bonus, as it would look gross. On a less nurgley model, I would thin with contrast medium rather than water, to avoid splotchiness and keep the effect more subtle).

My GUO did take a while to paint, but if you like it, I have some very good news for you: it didn't take very long and I didn't really do anything difficult or use any strange products or chemicals. Apart from the newly added effects, everything was done with a couple of brushes (some big cheap ones and my regular model brushes), regular GW acrylics, a couple of shades and contrasts, some sand and some Nurgle's rot for the base. I didn't do any fancy techniques like wetblending or the like either - in fact, many areas aren't really shaded or highlighted at all, if you look closely and I purposefully didn't even pick out all the little details (for artistic and laziness reasons).

Here's how he looked before the oils (regrettably he's upside down, but I was living in a no-gravity environment at the time...)

Assuming you like the result, I think the big thing to take away from my GUO is that painting models that are almost all skin/flesh doesn't need to be difficult at all, as long as you avoid the biggest pitfall of all: having all the skin be the same base colour, highlight and shade colour, because no matter how well you do it, it can easily come off as a bit boring and "plastic toy" looking.
As far as I'm concerned, the only real key to making a big figure like this look good is to have different colours mixed in here and there, so the overall result is varied - it will look much more "natural", even if you're not a master of placing shadows and highlights, simply because real skin is not the same colour everywhere on your body (well, I assume not, I can only speak for myself, of course...). We don't get the same problem with smaller minis, simply because they're smaller, so the skin being the same colour everywhere isn't as noticeable.
Here's another example:

Incidentally, we see the same thing with terrain, where painting it like you would a regular miniature sometimes ends up looking worse than doing something much simpler that just looks better at that scale. But more about that in another post 

And to finish off, a couple of shots of some poxwalkers I'm experimenting on. One has had an all-over wash of the same purple/magenta wash taht I used on the GUO, another has had a more targeted application and the last one hasn't had any at all. As you can see, oil washes seem to have a lot of potential for painting skin and gross features, but it's not really a necessity at all, as long as you're not too afraid of putting different colours into the skin areas.