Clive,
You say it is alright until you have done about 15 miles........this makes me wonder if it is the wrong grade / type of oil and once it gets hot, the problems start. It is doubtfull that the original specification was for an auto transmission fluid, probably used a straight grade engine oil. So do you know what it is filled with? If not, then suggest you renew the oil first, according to the vehicle's manual.
regards, Richard