The paint thinner has worked fine for me; but I did not soak the plastic parts very long. I just used the solvent to brush out the old grease and then dried them with rags.
Another couple of things to note is that you need to be careful not to lose the caged bearings and spacers; sometimes when you lift the drum off they stay stuck on the inside; but fall out of the drum at the most in-opportune moment and go in the drink! Same for the pawl springs, they tend to go flying when you take the pawls out.
This time I have chosen to remove all of the internals and winch drums, bring them home and do all of the cleaning and re-assembly of the pawls here. Then when I go back to the boat all I need to do is clean up the base, re-assemble and grease the gears and bearings. It will save me the hassle making a mess with solvents and oil soaked rags onboard. Last time despite being careful I had to clean up the mess of grease and thinner in the cockpit; every splash of solvent while brushing the gears makes an oily spot on the gelcoat. I know this is not an option if you are cruising but for those of us in marinas it can be easier to bag the parts and do the work at home.
Pawls should not have severely rounded corners, no chips, etc. Some mild rounding on the wear edge is OK. Always have replacement pawls, springs, clips aboard should you need to do a quick-fix. Lewmar sells a kit that has an extra plastic keeper for the shaft and a circlip for the top in addition to the pawls/springs.
it says "for winches less than 10 years old"; but I'm pretty sure that older winches can use this kit also. If in doubt call Defender customer service and ask for the correct kit for your winch.