NEVER NEVER use WD40 will stuff shit up!! On car or boat.
will dry hoses/elec casings out and make them brittle.
Inox Better.
Soak barings in light oil overnight.
Wash in warm soapy water first and dry thoroughly.

+1

Get some reel grease and oil from a tackle store (quantum do a pack with both though any half decent brand will do).  Thin coating of grease on the gears and other parts - oil on the bearings.

WD40 is actually not too bad for cleaning reels/reel parts, but pretty terrible if used as a lubricant/protectant (as above, use good grease and oil.)  I've never encountered WD40 degrading plastic, but I've also never left plastic soaking in it.  Shimano USA actually recommends a light misting of WD40 on the outside of a reel, waiting 10-15 minutes for the salt to rise, and then wiping it all down as the post-trip cleaning method for reels. 

I'm not a fan of Inox--FWIW I believe it also comes with a warning that it can have an impact on rubber--and would try Lanox (from the same company that makes Inox) or lanotec instead.  But really, just get some good grease and oil.  It can be helpful to take pictures as you take the reel apart if you aren't very familiar with it, and don't grease the anti-reverse bearing--just use some light oil.

I'd get a schematic and strip it if you are brave enough. WD40 is fine to use to clear away old grease/oil. I generally use WD40 for heavy cleaning, then soak in soapy water. Dry thoroughly! Then you can re-assemble using a quality grease and oil.

Don't make the mistake of smothering it grease.....a little grease, a little oil on appropriate parts. I use the Daiwa greases and oil - Daiwa grease is pretty heavy though so depending on reel you may want something else. There is a popular grease I forget the name though.

Don't put too much oil on bearings, a few drops should be sufficient. Oil the parts that need to move freely. Light grease for the inside mechanism. 

 If I have any reels that I pull apart and inspect / service, I generally clean all components with brake cleaner. Let that evaporate, oil all bearings, grease the gears, anything you are unsure of give a light coating of oil before assembly. 

Make sure you pull the drag washers out and clean them and the spool in the same way, and then apply a small amount of drag grease. Finally don't forget things like handle bearings, line roller bearings and also the bail arm spring assembly.

as mentioned earlier a schematic of the reel and YouTube will be your best friends as part of this process

scano

 If you do find corrosion on the gears, place the gears in bi carb soda, then cover with white vinegar (it will foam up a lot) this will help to eat away any corrosion evident on the gear faces.

scano