I put 80/90w gear oil in my rear diff. Is that alright? The truck is a 1977 Ford f150 351m C6.

Also another note, I was really low on oil back there, and I noticed after I topped it off the color of the oil that dripped out was black and it almost looked like it had formed some precipitate in it or something. Any thoughts on that?

 

Always check the manual. 80/90 gear oil should be OK. Is it a limited slip differential? Those require an additive. Gear oil gets nasty looking in a differential, and smells worse. It won't hurt anything to change it afaik. It's the sulfur in it that makes it stinky (I think) Wear old clothes, it won't wash out.

Most differentials that I'm familiar with, have a fill plug. On level ground the gear oil should be level or just under the lip where it goes. One knuckle of a finger say, is enough. Make sure vent tube is clear and unobstructed, and keep an eye out for any leaks at the axle. Oil doesn't evaporate, it went somewhere.

 

"Precipitate"? Let's try 'condensation' !

2X with 80/90.. the other option if towing alot is 75/140.

Yes, it is sulfur.. a high pressure lubricant because the molecules can withstand alot of shear before the covalent bonds separate. Ooops... maybe too much physics?

If it was low, 2x.. it's gotta go somewhere... Check for wetness at the axle seals (pull the drums), pinion seal, pumpkin gasket, vent fitting, filler plug.. unless it is experiencing alot of heat, burning, and is being expelled through the vent fitting and hose. You'd smell it though.. like rotten eggs.