The OM602 in the W124 does have slightly different oil cooler line specs than the W123 300D. The W123 used 16mm ID hose, but the W124 often needs 17mm or even 18mm depending on the barb style. 5/8" hydraulic hose is close to 16mm, but if it feels too tight, you risk damaging the hose or not seating it properly. Cohline 2134 is the OEM-style hose and worth getting—it’s rated for oil and high temps.