Yes, it’s possible to download PlanetScope 8-band surface reflectance (SR) images using Python, you should use ortho_analytic_8b_sr as the asset type when requesting 8-band surface reflectance data.

Here’s how you can filter for 8-band surface reflectance assets using Planet’s API with an AndFilter:

"filter": {
   "type":"AndFilter",
   "config":c
         {
         "type": "AssetFilter",
         "config": o
            "ortho_analytic_8b_sr"
           ]
         }
     ]
}

If you’re using Planet’s CLI, you can specify the asset type in your download command:

planet data download --item-type PSScene --asset-type ortho_analytic_8b_sr --date acquired gt 2020-04-01 --date acquired lt 2020-11-01 --geom your_geojson_file.geojson --dest your_destination_folder

I would also note that some imagery which is available in 4 band doesn’t have 8 band, so searching for assets that explicitly have 8 band is important (such as older images, before SuperDove, or images which don’t have 8-band calibration)