The LN3 was a mixed brew when it came to valve covers. The valve covers in my 1989 Lesabre has metal valve covers -painted. The LN3 3800 I pulled out of my 1989 Lesabre parts car and put in my 1990 Bonneville SSE had plastic valve covers.
The motor that came out of the 1990 SSE had metal covers -painted.
My dads 1990 Olds 98 has plastic covers. My sisters 1989 Lesabre Limited has plastic covers. Another sister has another 1989 Lesabre Limite (my first Lesabre) and that has the metal covers.
I pulled the plastic covers off my 1992 Bonneville engine and they've been sitting around the garage and so when I swapped the motors in the 1990 SSE I pulled a valve cover and aside from being different materials they appear to be identical as far as dimension/bolt locations goes.
Sorry to go off-topic.