Thinning Yellow Waterproof Wood Glue?

I have in my shop a gallon jug of the yellow waterproof casein wood glue used by professionals. I am a professional wooden body restorer of early brass era antique autos, but I know nothing about revitalizing 6 year old glue which is thickening in the jug.

It is like syrupy, slow pouring out of the bottle, and very sticky. There are no clumps nor lumps. It still dries and glues like normal after application. It takes lots of soap to get your fingers clean after use. The fella I use to get it from died 4 years ago. He purchased it in 5 gallon pails. He manufactured those Georgian columns for homes. Some of those were 3 feet in diameter (each) x 15 to 20 foot tall. I want to use the old glue as it works. But I need it thinned down without harming the adhesion. Anyone know how to do this?