180 -190F IT for slicing.
195-203F IT for pulling. Start probing for tenderness at 195F IT.
If there is enough fat, which there should be, injecting may not be necessary.
But if you want flavor down into the meat, then inject away.
Think along the lines of lemon, garlic, thyme, sage, rosemary, etc
Or use a store bought herb marinade.
You might need to run the marinade through a blender so that the larger spice pieces don't plug up the injector needle.
Wrap it foil and let it set for 10-20 minutes before pulling.
Are you planning on using a rub as well?