What oil and air filter to use in 1988 90Q?

[Radek]

What oil filter and air filter can I use in my 90 Quattro, 1988, other than
what's available from the dealer $$$?

Also, what gearbox oil (manual) and differential oil to use (climate:
Canada).

The car has 220 000 km (about 140 000 miles).  Is it OK to use syntetic oils
in a car this old (I heard opinions that seals will start leaking because of
the finer molecular structure of synthetic oils).
Any suggestions will be appreciated.  And again, should someone have a
Bentley manual available, please let me know.
Thanks to all.
Radek.