which rear end lube?

circle track 63 dart with 8&3/4 5:57 with welded spiders. I went to O'reillys to return some rented tools (what a gift/much appreciated) & bought a gallon of 75-140 non synthetic gear oil. the manager told me that synthetic would be a better choice as it dissipates heat better & should I stay with this heaviest wt of lube (there were lighter grades on the shelf). thank you for your time. RR