From what I remember,the red cap was discontinued and the gold cap remained. The gold cap was then discontinued (Target clearanced theirs out at a dollar a quart,so I cleaned the shelf!). For awhile M1 15W50 was completely discontinued and apparently it caused a huge uproar between loyal customers and Mobil. Mobil then had a banner on their website that said "Back by popular demand,Mobil 1 15W50" (which then became knows as Mobil 1 15W50 silver cap).
Mobil 1 15W50 seems to be legendary and has a kind've "cult following".