I've switched to a heavier differential fluid to reduce transfer case leakage. For some reason, I have both Redline 75w140 and 80w140. I have a differential/transfer case fluid change coming up soon. My assumption is that I could use them together. Any concerns? And which is preferable going forward.
Thank you.

 

Given your location go with the heavier weight. Minimal difference anyway but in SC the heavier will leak slightly less, cushion slightly more.

 

makes zero difference