"Y" is a dry(some recent vintages may have been off-dry) white Bordeaux made by Chateau d'Yquem in a few years. It can not be classified as Sauternes, because it is not made in the manner required by law to be entitled to this name.
The bottle of 1978 "Y" had been properly stored since release and had a low neck fill and good cork. Michael Broadbent had a bit of information about this wine in a recent book. He said it would be interesting to see how this wine has evolved, since his last tasting was in 1990. He did state that the wine was made from 50% Semillon and 50% Sauvignon Blanc. It all went into 100% new oak barrels, as does Yquem, and it was bottled in March 1981.
The color is now bright yellow with some gold - not unlike Yquem of about this age. There is no trace of oxidation. The grapes must have been very ripe, but there is no sweetness. Think of Yquem with the botrytis and honey-like character removed to get a gross general idea of the character of the wine. There is sort of a creamy honeycomb character rather than honey. You can taste and smell hints of Sauvignon Blanc if you concentrate, but there is no strong "tomcat" character. There is enough acid to balance, the wine is now quite smooth(it can be a bit raw when young). Despite use of 100% oak barrels and about 2 1/2 years before bottling, there is no excess of oak because the wine is big and concentrated and can handle the oak. There is very high extract and likely high alcohol. This is a wine that can stand up to very rich dishes and sauces. I had it with large crab cakes made with a lot of cream rather than the more usual US crab cake that uses mayonnaise. It was served with a lobster sauce, also loaded with cream. I think I would rather have this wine with foie gras than Yquem or other very sweet wines. It would be interesting to slip a bottle of mature "Y" into a blind tasting and see what happens.
A more conventional dry white Bordeaux is Chateau Laville Haut-Brion of which I have had the 1978 recently. I find it a somewhat better wine than the 1978 "Y" with a bit better balance and more complexity. It is not at all unusual for a top dry white Bordeaux such as this to last as long as many top red Bordeauxs, despite the much shorter life of many Bordeaux whites. However a white Bordeaux that has become oxidized can be very nasty tasting indeed.