The old civic/ballade/integra and the accord/vigor manuals tell you so, The Delo Gold (old name Delo 500) product is chosen because it has excellent anti foaming charecteristics, is quite stout and readily available in Pakistan. Yes it was 15w40 - diesel engine oil.
Later on when Honda started selling manual trans fluid - they claimed to only use theirs (marketing). If I try to count the number kms those same transmissions have done then it would be quite astronomical - every single one never faltered and every single one that was changed out from some ghaslty 140 gear oil to Delo gold was a smooth runner, infact it even gave better fuel economy as there was less drag.
Not all transmissions take gear oil, - e.g. MB manual transmissions take automatic transmission fluid and they work flawlessly, the ZF units used in some land rovers are also like that.