This is a huge topic, and believe me, nobody has all the answers. Not even me. Especially not me... I've seen too much! So take everything I say here with a grain of salt and apply common sense. TEST TEST TEST! Did I mention, TEST?
People have lots of opinions about what works to clean the grunge off machines, inside and out. If it works for them, it's correct. I'll tell you what has worked for me.
Personally, I like the old standby, kerosene, for cleaning innards. Kerosene is oil, a thin oil, but nevertheless an oil. Thus it very likely isn't going to destroy anything (except maybe your lungs if you sit with your nose in it day after day after day; ask me how I know). As a rule, oil will harm neither metal parts nor decals. Plastic parts, I don't know about. I mostly work on old, all-metal machines, so anything with plastic gears and/or parts, I am not offering an opinion. Fair warning.
I clean old oil and grease off using kerosene on a toothbrush. Parts I can remove, I put in a kerosene bath, let them soak a bit, and scrub with the toothbrush. No big issue about getting it all dried off because it's oil.
By the way, WD-40 is a water displacement (WD) product, not a lubricant. Please don't use it to lubricate sewing machines. Over time, it will dry to a glue-like substance that will cause your machine to gum up, and require a major clean-up operation to un-do.
If parts are frozen stuck, e.g, screws that won't unscrew, pieces that won't move, my favorite product is Kroil. Like kerosene, Kroil is an oil, so it's not likely to hurt anything. Unlike kerosene, Kroil's molecules are microscopic, so they can creep into the smallest of spaces, which is how it loosens frozen parts. There are other products that claim to do this, but in my experience, it's apples and oranges. To each his own. I have no vested interest in any of these products, just telling you what I've experienced.
Bright parts (metal and chrome) can be cleaned with metal polish. My personal favorite is Mother's "Mag & Aluminum Polish."
Okay, the big question: How to clean the outside? A lot of people say sewing-machine oil. Hmmm. That'll attract a certain amount of dirt to the cleaning rag, I'll admit. And shine things up. But cleaning? I don't think so.
There are folks who like GoJo No Pumice Formula. Personally, I have found it to clean, but it also silvers decals. The Green and the Orange cleaners I have found to be too harsh, and also to silver decals.
I've tried the Meguire's three-part cleaning-polishing-waxing system (available in auto stores) and liked it really quite a lot... better than anything else I'd used prior to that.
But then I found Blue Magic's TR-3 Resin Glaze. Let me remind you, I have no vested interest--no interest whatsoever, they don't know I exist--in any of these companies. Just telling you my experience. And my experience with TR-3 Resin Glaze has been as close to magic as Kroil. I used it on this machine:
I forgot to take a "before picture of this one, but I assure you, it was completely brown, the decals were faint intimations that something might be there, at best.
Note: You can't bring a machine back to this condition if the finish is compromised under the grime. No product is that magical.
I did think to take a couple picture of this one. It is a Singer 301, black long-bed, that I figured was going to be a "parts" machine because it was cosmetically gone. I was thinking acid or something had been thrown on it, damaging the black paint permanently. Tried a bunch of other products before the TR-3. They did nada. This is what I had:
It's actually not a good picture of what it looked like... imagine those white spots and dribbles really accentuated, they were really white... more like this photo of the bed:
I'm talking seriously disfigured. Nothing apparent on the surface, like paint. I figured some chemical transformation. And then I did the TR-3 treatment (after using some other stuff, that didn't work). It took a third of a can, and a whole bag of cottonballs, but hey, look:
That's as close to magic as it gets, in my book.