How often do I change differential oil?
So I bought my 1984 F150 a couple months ago and did all of the regular maintenance. But I occurred to me that I didn't do anything about lubing the 4x4 linkage. I've recently actually engaged 4 wheel drive for the first time and there was a little more mechanical noise than I feel is normal.
How often should I pop those differentials open and change their oil? And should I have done that already, not knowing when the last guy might have done it last, or is there a way to check the quality of it?