Your $0.02 Please

Since Denver gets cold in the winter,(at least it used to) and the high mountain valleys get *really* cold, I have been considering taking advantage of the greater viscosity spread offered by synthetic gear oils for my 1986 300SDL. There is a product, Valvoline, I think, that offers a full synthetic GL-5 in the 75W-90 weight. I like the cold weather lubricity offered by the 75W and the hot weather protection offered by the -90. Any thoughts? Are there differential seal issues with synthetics as there are with engine seals? Frankly, the seal issue is probably my greatest concern.

Thanks for all replies.