My uncle has a problem with his EF I6. On start up only there is a puff of smoke. I suspect valve stem seals letting oil seep into cylinder over night. He was using Castrol magnatec oil (5w something) I told him its way too thin. He replaced the oil with the correct grade (15w40) The engine has done 230,000 k's The idle is fine and no sign of oil in the water and vise versa so doeasnt appear to be head gasket. If it is valve stem seals can they be replaced without removing the head (pressurising cylinder with air so valves dont drop into cylinder) Any help or advise would be great thanks.

Probably valve seals. Follow him and see if it puffs when he backs off throttle down along hill.

Can be changed in car but if you are not sure then be careful as the two biggest problems are dropping components down into sump and dropping valves into cylinder.

Put "valve seals" and "valves" into search engine to bring up info.

Grumbles