15W50 is fine.

However, you appear to have a bad visco clutch on the fan -- temps shouldn't get that high unless you are in the tropics this time of year (or in Australia) -- my TE got hot until I replaced the radiator cap and the visco clutch -- problem solved.

If the noise is a definite slap, get it checked out pronto by someone who knows these engines even if you have to take a trip to do it -- bent valves will set out back at least $4000 USD!

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!