Hi Thomas, afraid not exactly an old wives tale.   Here is an extract from an  old NACA report that appears to take the position that submerged (NACA?) ducts are not the best suited for radiators or anything requiring much diffusion or pressure recovery. Unfortunately,  I don't have the full report but the report number is listed on the summary below if you wish to try and find it.   Any extract can be misinterpreted taken out of the context of the total report - but, it seems to take a position.