I have double glazed windows of the (excellent) old type which involves 2
panes separated by a 10cm air gap. They are all about 30 years old - when
the house was built. These windows swivel on a horizontal axis for
ventilation and cleaning. Most of them swivel easily and stay in any
position but some are either very stiff of very loose, the latter cannot
stay open.

They are marked "PFI" but there are no other indications as to make or type.
Each hinge seems to consist of 2 D-shaped pairs of plates. There are a
number of alan key adjusters but I cannot find combinations which improve
the mal-functioning hinges. One particularly stiff one presumably would be
helped by some form of lubrication - but what oil, grease?

Anybody got experience of these things and can share it or know where to get
useful information.

Frank