San Tee or Wye when tieing in new drain

Whan adding a new drain to existing stack should it be tied in with some form of a "T" or a Wye connector?

connection

Either one MAY be acceptable, but it depends on a lot of other factors that we don't know about.
1. What kind of stack is it?
2. Are there other fixtures above your proposed connection?
3. Will that connection need its own vent?
4. Etc.

This existing stack is really just an existing clothes washer drain that goes directly down a 2" pipe into a grey water drain field. I am looking to add a shower to this drain field. The existing shower drain is vented now and works fine so I was planning on keeping that. I was hoping to just tie into this vertical drain and thinking that 2" would be a sufficient drain. The only thing with the washer drain is that there is no P trap or vent but is seems to work.