THE needle to use is the one that came with the OS 10FSR. Fine threads, easy adjustment. Most of the needles used in the old OS FP series engines would work, too. If there's an air leak in the needle assembly, the fuel leaks out rather than air leaking in. Unless the fuel line after the needle doesn't fit well.

If you have a leak, seal it by putting a short length of neoprene tubing around the needle. Or use teflon tape.