I have a Karu 12, but I’d assume the fuel box is of similar size.

I’ve found it works best to get the fire going with lump charcoal, then add a hardwood to produce more flame that will be pulled over the top of the pizza by the oven. When I start my fire, I put a few fire starters (GreenSpark 100% Natural Firestarter, Fast Lighting, Long Burn, Waterproof, Safe for Indoor/Outdoor Use, 70 Count https://www.amazon.com/dp/B07FPQVLFH/ref=cm_sw_r_cp_api_glt_fabc_TE7623JTBM8BYEXAC1EB?_encoding=UTF8&psc=1) and then fill the box with hickory or oak lump charcoal.

I purchased a few bags of oak chunks from a local firewood shop, then cut them to smaller pieces using a bandsaw. Even the chunk Home Depot sells is a bit big for the Ooni. If you don’t have access to a way to resaw the wood into smaller chunks, I’d probably recommend just getting the stuff from Ooni.

When using the lump charcoal plus wood fuel, you don’t need too much hardwood. I usually add 1 or 2 pieces right before I start to press out my dough so that they are fully burning when I toss the pie in. Depending on how many pizzas I’m making on a given day, I’ll add more charcoal over the course of the evening to keep my fire nice and hot.

Highly highly recommend oak or hickory as wood, great heat and a bit less smoke than mesquite.