Quicksilver is only an aftermarket subsidiary of Mercury. There oils are likely the same, and when you buy Quicksilver parts you will notice they are considerably cheaper than Merc parts.

I would recommend the Quicksilver parts and oils for anyone who can't find Merc brand parts, or anyone trying to pinch some pennies.

I probably worked at one of those dealerships that you spoke of that shut down, and we carried a lot of Quicksilver parts. When performing services we used Quicksilver filters, oils, impellers, etc -- unless the customer made it clear that they wanted Merc brand parts.