international recommends an 85w140 but I cant find a full synthetic at that exact weight. would there be any detriment in using mobil1 75w140 instead? as far as I understand the 140 is the main number the other is flow rate while cold which may help it during winter a bit or is there more to the cold weight than that?
would you recommend mobil1 synthetic or some other brand for this rear axle and diff?
Thank You ahead of time for your suggestions and insights!
Why do you want/need a full synthetic? Are you doing long hauls and changing every 100K miles?
I wouldnt want to spend a bunch of money on synthetic rear diff oils and have a wheel seal blow, and have to chase down a synthetic.
Your 1986 S1900 will not last any longer or run any better on full synthetics..
Put your efforts towards good oil filters, fuel filters and air filters. Check fluids often.. run it and keep an eye on it.