Last night I made a vinaigrette with walnut oil, a bit of black truffle oil, red wine vinegar, a bit of dijon mustard and S&P. On assembly the texture had a nice viscosity, but I didn't use it to dress the salad until about a half hour later. By that time the dressing had become as thin as water, no amount of whisking would return it to a thicker texture that would cling to the leaves. Obviously next time I'll make the dressing at the very last minute, but I was topping the salad with poached eggs and wanted the eggs to be the last step. Has this happened to anyone else? Why did my vinaigrette become as thin as water?