300TE low oil pressure - oil pump replace ?

[hofflandmoss]

Recently my 1989 300TE was showing 1/2 bar at road speeds. Mechanic checked, 4 PSI at idle, 31 PSI at 3000 (hot)with 5W30 . Pressure erratic, he said it acts like a broken relief spring in oil pump - says probably need new oil pump. Not sure he can raise engine enough to get oil pan off - if can, says $2000. If has to remove engine, additional $1500. Car has 300K miles but has been well maintained and other than some play in steering drives well. Is new oil pump needed, and if so, does $$ seem right ? Is spending this amount worth it ? Would going to heavier oil help ?