Hi looking for some guidance. We have a 1920's house with 1x6 plank subfloor looks to be rough sawn with maybe 1/4 inch gaps between the boards. It is mostly in good shape and seems sturdy. Joists are 2x6 16" on center. We want to install 5" width unfinished hardwood (maybe brazilian walnut or we may stick with white oak). Should I glue down due to the width? Will this subfloor suffice or if not what do you recommend? I was planning on at the very least screwing it down. If climate matters we are in southern CA.

The NWFA recommends glue and fasteners for 5 inch and wider. I just use staples. It's your choice. A staple gun using 2 inch staples with the base removed can help secure the sub-floor with a staple between the 8d nails. A moisture barrier under the floor or Aquabar Kraft paper over the sub-floor is good, depending on whether you like to glue, also. The sub-floor sounds good, but be sure to flatten the floor by sanding or planing the high spots. Build up low areas with three tab asphalt shingles. Don't run the flooring the same direction as the sub-floor. Very few sub-floors are ready for flooring without preparation.
If you have to use screws, make sure there isa smooth shank at least 3/4 inch long before the threads, so you actually pull loose spots securely to the joist.

Great thanks appreciate it. So maybe dumb question if I am nailing and using glue what underlayment would I use for the subfloor as the moisture barrier?

Quarter inch exterior glue plywood would be fine.

Thanks not sure if I should post this as a new question but thought I'd continue this. We plan on going with white oak select and better grade and decided to go a little less wider so probably 3 1/4 wide. I will be staining it and then plan on using waterlox to seal it. Should I trowel fill? If so considering that I'll have stain is there a best practice for this for the filler to match while using an oil sealer?

Trowel filling I only use with small pieces of parquet flooring. A flexible putty knife and wood patch works for me on strip or plank flooring. Buy the wood patch for the specie that you are going to install. It ought to be close in color to the darker boards. Most wood will darken over time with exposure to light.