Hey again,
Not really a lubricant for that..and anything that you may use could eat away or damage the belt over time.. your belt # should be a 49-083... model #s prior to DT-600 took a #925-01-032-1060 which is no longer avail...you might try to keep putting the belt on the one side as far as it will go and then take off turn to the other side and install..this may stretch the belt enough to go on all the way...I know this is tough as i have torn belts in the past installing them..! UGH [make sure you do have the correct belt first as dale suggests]
good luck...!!
Craig