...It requires GL5 70w 90 synthetic gear oil...
I think you have already answered your own question!
I WOULD NOT use conventional oil on a drive which calls for synthetic.
Are you willing to risk a $3,000 drive over a couple of bucks difference per bottle?
I'd do what the factory manual calls for and have piece of mind.
I used to use the Mobil 1 in my drives but have now just gone to using the Volvo oil since all the Mobil 1 bottles I see mention LS.Also, as mentioned in other threads, avoid LS (limited slip) gear oils on the Volvo and bravo drives with cone clutches.