Reddit mentions of Maruman Mnemosyne Inspiration 5 mm Grid 6.3 x 8.3"

I'm going to start off with a very quick "don't buy" recommendation - Moleskine. The paper is terrible for fountain pen use.

Now, on to the good stuff.

I don't think you can go far wrong with Rhodia Webnotebooks. Some complain of feathering, though I suspect they may be using broader nibs with wetter inks. I've seen others complain about ghosting, possibly due to the same reason. Ghosting hasn't a problem for me though that may be because I prefer EF or F nibs.

Or the Leuchtturm1917 range. Another classic FP friendly notebook.

Both are readily available from stationery stores and online. They're great notebooks. And with the elastic that they each have, you have a readily available pen holder too. (Though maybe not as a permanent solution for your fanciest, most expensive pens.)

Spread your wings a little wider and you might want to think about something a little less mainstream. I really enjoy how it feels to write in a Maruman Menomosyne "Inspiration" pad. It is one of my favourite papers to write on, though a touch too spendy for me to feel comfortable using it for my daily scrawling.

Then there's Midori. The paper in their infamous Travelers Notebook refills is well regarded. However you need to watch out for getting oils from your hand on the paper and, occasionally, there are pages where the paper finish hasn't fully taken. In both cases you can see excessive feathering and ink 'burps'.

Around the midpoint of last year I tried the Midori MD Cotton Notebook in A5. That replaced the Leuchtteurm that had been with me fro four or five months and stuck with me for the rest of the year. I'd be well into my second if it hadn't been for the next, and final, recommendation.

Last, but not least, my current daily writer is the Tomoe River based Hobonichi Cousin. I believe that this is the thinnest FP friendly paper you can find. It ghosts, but with paper this thin I'm not sure what else you would expect. And it can take a little longer to dry than the other options here. It is definitely a paper that you want to consider carefully if you don't want to hang around a little waiting for ink to dry. Blotter paper, or a pencil board/writing mat, may help. (I use a pencil board and, when I'm in a rush, drop it in between the pages I was just writing on. I haven't suffered any disastrous smudges yet.)

It is a dream to write on and, being so thin, you get a serious amount of pages in a small amount of space.

The Hobonichi isn't to everyone's taste. And we're almost halfway through the year so it's a bad time to invest in one. Instead, look at the Nanami Seven Seas Tomoe River Noteook. Same paper, same writing experience. I may end up with one of these next year instead of a Hobonichi.

Hope that has helped. Whatever you end up with I hope you enjoy it.

If you only use one side of the page Mnemosyne has a great a5 graph (the backs are blank though). Their notebooks have survived really well with me: Maruman Mnemosyne Inspiration 5 mm Grid 6.3 x 8.3" https://www.amazon.com/dp/B001A1VB74/ref=cm_sw_r_cp_api_mHp9Bb7XTNWN8

Maruman also has a student-ish line of board backed graph notebooks:
Maruman A5 spiral notebook grid ruled 80 sheets N247ES 5 volume set https://www.amazon.com/dp/B00MMW8M7M/ref=cm_sw_r_cp_api_3Hp9Bb1GSDT60

Rollbahn seems to be really high quality and the rings are no joke. They’re not hugely thick though so you may need a few for the year (unless you just do weeklies):
Roruban Pocket Memo A5 Dark Blue https://www.amazon.com/dp/B007B8N2YA/ref=cm_sw_r_cp_api_GIp9BbZ2AXW0S

I really love the nockco a5 spiral top bounds, smaller dot dash and easily landscapable. A little less sturdy: https://nockco.com/paper/dotdash-spiral-pad-2-pack

The Studioneat Panobook is supposed to be amazing. I cannot offer a personal opinion as mines still in the mail: https://www.studioneat.com/products/panobook

You could also look into a notebook case to reinforce a flimsier option you like (such as the Rhodia): KOKUYO SYSTEMIC A5 ノ−V685B−M GRAY https://www.amazon.com/dp/B00S0GQS3O/ref=cm_sw_r_cp_api_NMp9BbFNEMBQF

If you don’t need spiral bound I’d suggest:
Clairfontaine essentials book
Rhodia goalbook
Anything Life
Anything Apica CD

I personally really prefer spirals so they truly lay flat, so my day to day I tend to use my mnemosynes the most, I just dig the paper a lot.

Maruman Mnemosyne Inspiration 5 mm Grid 6.3 x 8.3" https://www.amazon.com/dp/B001A1VB74/ref=cm_sw_r_cp_api_v.qfAb5FQFHAJ

Grab a Pilot Plumix with a Medium Cursive Italic nib and swap that nib into your Metro (use the Plumix body to hold your spare nib and hide it in a drawer).

http://www.amazon.com/Pilot-Plumix-Fountain-Barrel-Medium/dp/B00A6VW87G/ref=sr_1_2?ie=UTF8&qid=1449781768&sr=8-2&keywords=pilot+plumix+italic

Also, I'm really liking this Mnemosyne notebook for carrying to meetings where there's not much desk space. Really great paper.

http://www.amazon.com/Maruman-Mnemosyne-Inspiration-Grid-6-3/dp/B001A1VB74/ref=sr_1_4?ie=UTF8&qid=1449781799&sr=8-4&keywords=mnemosyne