I'm not a big fan of sumo lifts (unless called for in the WOD) or straight legged DLs. I'd like to focus on form and consistentency of the movement so I can incorporate it into my Oly-lifts.
The Romanian DL is more specific to Oly lifts, but even that can catch your knees a little. I think that the most "pain free" substitute for the DL would be a good morning. A great exercise, but it obviously lacks some components of the DL, grip strength being one. Still, it may be a decent substitute if your gym doesn't allow DLs. As far as carryover to OL, my lifts have improved since I started doing GMs, but your mileage may vary.