Dale, I'm not sure what was going on in your truck but I can assure you that the base ingredients in Mobil 1 grades are fully miscible. When 2 fluids are miscible they will not separate on sitting. A good analogy would be a bottle of Bourbon. The alcohol and water will not separate no matter how long the bottle sits.
On the other hand, I have observed that in a 1 gallon jug of Delvac 1, the last material out of the jug can be a little cloudy. I attribute this to some of the additives separating.
You may be interested to know that Mobil1 10W-30 has a higher specific gravity than 15W-50 (0.872 vs 0.863 respectivley.) the specific gravity of 5W-30 (0.862) is almost exactly the same as 15W-50. It's only a guess but I think that specific gravity decreases as the level of additives increases.
1984 300D Euro