Synthetic gear oil in differential

Since purchasing my 1971 CJ5 I have noticed that there is a very sight gear oil leak from the differential to the right fron axle. When I checked the oil in the differential it was a bit low. It was a clear somewhat thin oil. It did not match the rear differential gear oil, which was thick and brown. I haved never seen synthetic gear oil. Does it sound like the front could contain synthetic? I've heard that it has a tendency to cause leaks. Will replacing it with standard gear oil stop the leaking?

 

In fact a lot of synthetics contain elements to recondition (ie.,swell) seals and will prevent leaks rather than cause them. Clean looking or clear oil in the front could very well just be it has never been run in 4wd. That is the only way an oil will dirty up (change color). It tends to go black in the engine from combustion products and a brown to black in gear cases from wear and dirt (or kinda orange from water and corrosion).:v6:

 

I was of the understanding that because of the properties of synthetic lubricants that if a gasket was weak that there was more of a chance that it would leak with synthetics than with mineral lubricants.

 

I run synthetic in my rear diff and have been for at least 5 years with no leaks. However, I didn't have the same results using it in the 18. The yoke seals were fine but it would seep out on the shifter shafts even after replacing the seals. I went back to the mineral based gear lube in it and have not had a problem since. My take is that it can induce leaks, in my case perhaps due to wear/pits on the shifter shafts.

 

As a follow up, I assume the 71 still has the 27 axle. There is little to keep the diff lube from flowing down the axle tube and into the king pin knuckle and usually, shortly thereafter, through the wheels seals and into the brakes. The problem is exacerbated by over filling and/or doing steep side hills. I typically leave the level 1/2 to 3/4 inch below the fill plug which may be what the PO did with yours. I still have my stock front axle and other than beefing the shafts and changing the ring and pinion gears, it has never had maintence problems-still runing the original bearings.