My '88 300TE with 103 engine has done 265k miles with no attention to valves. I am the 2nd owner & have detailed service records from 11k. Timing cover oil seals done at 128k (still weep a bit though). I replaced the hydraulic lifters at 132k due to a couple of noisy ones. Otherwise only routine servicing ( oil & filter at c6000m). No visible smoke from exhaust though it uses about a litre of oil every 1000 miles or so now. Plugs stay clean between changes and it never misfires or fails to start first touch. Still runs quiet as a mouse and has as much power as my 300TE-24 (with 104 engine) up to about 4500 rpm. The 104 is far noisier, thirstier and temperamental.
Give me a 103 in preference to a 104 any day!
90 300TE 24v (M104 eng) 207K miles
87 300TE (M103 eng) 269K miles (and never had the head off!)
86 300TE (M103 eng) 230K+ miles (donor car)
95 Toyota Lucida diesel 4wd
86 200T 165K miles (sold)