I'm not a Lucas fan at all, though I have never used their gear oil and my opinion is based on their "some thick [censored] will fix it" philosophy with their "oil stabilizer," ATF molasses, and PS molasses. Lucas products are for wringing the last bit of life out of something that should already be in the junkyard anyway.
Any brand of synthetic is fine to use. I have used Mobil 1, Valvoline, Motorcraft, Royal Purple, and even Super Tech in Ford 7.5s, 8.8s, and a Dana 35 and have never had any differential problems ever aside from expected pinion seal leaks (both of which happened to seals with over 150K miles on them and lots of exposure to mud).