Rear diff fluid weight????

I looked at the sticker stuck to the underneath of my hood, yes my hood still has all the original spec stickers on it. It states that for the rear diff fluid its straight 90w or straight 80w depending on the ambient temperature. Would synthetic 75w-90 be sufficient? Technically it covers both temperature requirements. Thanks for any advice.

I've always used 80W-90 gear oil for both differential and the manual transmission on my '88 B2200. 215K on it now.

I've changed both of those fluids out once before but can't for the life of me remember which weight I used. Its frustrating getting old, can't remember shit. My B has 242,000 miles on it.