Well, a simple enough question but it gets complicated. Typically you will find the chain is Grade L. Its galvanised lifting chain spec. Its OK except that the galvanising is relatively thin, and you find you have rusty chain all too soon. Having said that you could expect 3-4 years life if you anchor a fair bit. Provided you don't paint it for depth markers etc then you can later get it hot dip galvanised to increase life.
My last chain was 10mm CMP, a 110m drum, sourced from Marine Trade Supplies. Originally I ordered 8mm G43 chain from them. But on the way down for haulout my old Ideal windlass wasn't working properly after several efforts to fix it. So I decided to replace it with a Muir windlass that had a 10mm gypsy. I went with 10mm grade L - from memory it was rated about the same as 8mm G43. I'm due for new chain and will most likely order from Muirs at Coomera prior to next haulout. They can supply longer lengths from their large (200 litre) drums.
Given that you currently have 8mm chain, and thus an 8mm gypsy, you might want to consider 8mm G43 to avoid having to source a new gypsy. I don't know how well the galvanising on it will hold up, and if you need to get it hot dipped in due course you will lose some of its strength, I believe about 10%.
You could also buy from Serafini Chains. They are a local manufacturer (Bowen Hills, Brisbane) so you can get whatever length you want. I've not discussed thicker galvanising with them, but it might be possible. Let me know what they say if you discuss that with them!
