Is there any official recommendation what type of connector I should use in Extreme SFP modules? I suppose I should use LC UPC, but can I use LC APC? I could not find any statement in Extreme documentation but for example Cisco doesn't support LC APC in their modules.
For my knowledge (its a long time ago),
The protective "glass" surface in a transceiver module (GBIC, SFP, SFP+, ...) is always flat (angle of 90°), so for the best contact (minimum loss) use LC-UPC connector types.
The Single-Mode LC-APC, SC-APC, ... connector types give a better result when connecting fiber to fiber, due to the angled surface there is less refection back to the transmitter, maybe the loss is also less compared to ..-UPC connectors.
For Multi-Mode it is always ..-UPC.