If you specify w: 0 but include j: true, the j: true prevails to request acknowledgment from the standalone mongod or the primary of a replica set.

 ?minPoolSize=15&maxPoolSize=45&connectTimeoutMS=30000&socketTimeoutMS=60000&wtimeoutMS=300&retryWrites=true&ssl=false&authMechanism=SCRAM-SHA-1&w=0&journal=true 

java.lang.IllegalArgumentException: state should be: journal is false when w is 0 

>>> uri="mongodb://root:rootpass@127.0.0.1:27017/admin?authMechanism=SCRAM-SHA-1&w=0&journal=true"
>>> 
>>> 
>>> conn = mc(uri)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/darshanj/anaconda3/lib/python3.7/site-packages/pymongo/mongo_client.py", line 692, in __init__
    username, password, dbase, opts)
  File "/Users/darshanj/anaconda3/lib/python3.7/site-packages/pymongo/client_options.py", line 164, in __init__
    self.__write_concern = _parse_write_concern(options)
  File "/Users/darshanj/anaconda3/lib/python3.7/site-packages/pymongo/client_options.py", line 61, in _parse_write_concern
    return WriteConcern(concern, wtimeout, j, fsync)
  File "/Users/darshanj/anaconda3/lib/python3.7/site-packages/pymongo/write_concern.py", line 76, in __init__
    raise ConfigurationError("Cannot set w to 0 and j to True")
pymongo.errors.ConfigurationError: Cannot set w to 0 and j to True
>>> pymongo.version
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'pymongo' is not defined
>>>

MacBook-Pro-52:myApp darshanj$ mongo "mongodb://root:rootpass@127.0.0.1:27017/admin?authMechanism=SCRAM-SHA-1&w=0&journal=true"
MongoDB shell version v4.2.8
connecting to: mongodb://127.0.0.1:27017/admin?authMechanism=SCRAM-SHA-1&compressors=disabled&gssapiServiceName=mongodb&journal=true&w=0
Implicit session: session { "id" : UUID("d744cd39-b32d-488c-888d-c4de0b2f0d68") }
MongoDB server version: 4.2.11
Server has startup warnings: 
2020-12-14T11:47:59.151+0530 I  CONTROL  [initandlisten] 
2020-12-14T11:47:59.158+0530 I  CONTROL  [initandlisten] ** WARNING: soft rlimits too low. Number of files is 256, should be at least 1000
MongoDB Enterprise > show dbs
admin   0.000GB
config  0.000GB
local   0.000GB
mydb    0.000GB
test    0.004GB
MongoDB Enterprise > use mydb
switched to db mydb
MongoDB Enterprise > db.mytab.insert({a:"m"})
WriteResult({ "nInserted" : 1 })
MongoDB Enterprise >