Yes, in mid-'99 valve size was changed, read here: Ford Ranger II 3.0 liter Engines
You can pull off a valve cover and see what you have, you can tell by the shape of the valve spring.
'91 3.0l heads would have 8mm, but a previous owner could have put on later model heads(or swapped whole engine), and if mechanic didn't know the shape difference and just went on 1991 model year there could have been a goof up.
You can change valve guide seals with heads in place, takes some time(1 cylinder, 2 valves at a time) and a smaller valve spring compressor, but no special knowledge is needed.
Other sources for oil on start up is PCV system
And just a heads up in case whole engine was swapped, '99 and up used longer head bolts, head was the same dimensions, the block was what changed, threads for head bolts were recessed into the block, shorter bolts will reach but won't hold very well