GMD-
Most drivers consider the engine lubricant oil cooler lines most problemmatic.
I can't advise where is best replacement ... i can caution that whether you go OEM or aftermarket, be VERY careful how the lines are routed and supported. Occasionally the OEM connectors at the block/oil-filterpad let go, and sometimes abrasion in vicinity of enginemount causes catastrophic (or nuisance) leak. But the chronic pain is the aluminum crimp collar at the hose/tube junctions relaxes, yielding drooling all over front end....
Some drivers have replaced both the engine and transmission oil-cooler lines with oversized.
tom m
'95 6.5td 4L80E 3500/srw p/u