oligo hybridization conditions for northern

[Josmar]

Dear all,
does anyone have the buffer conditions handy to hybridize hot oligo
probes (~20-mers) on RNA blotted and UV-crosslinked to nylon membranes.
I would assume conditions should be pretty native, as they would be in
a PCR. Unincorporated hot gamma-ATP is removed by gelfiltration. What
would be best way to block the membrane against unspecific binding of
the probe? Salmon sperm DNA?
Thanks
Josmar