Those old enough to remember may recall when a "tune-up" consisted of a lot more than just replacing the spark plugs every 100K miles!
Setting the timing and dwell, after replacing the plugs, points, cap, rotor & condenser (there'll always be those dual point distributors to fool around with) was a twice yearly chore if you drove a little... And adjust the carb, too. After you "read" the old plugs.
Remember how wondrous it was when everything worked?
Leaving plugs in for much over 1/2 the very long intervals the mfgs. now recommend is risky IMHO. Plugs are cheap! How better to find out what's going on in the combustion chambers?
Even on the SVX, where Subaru dealers charge 6 X $15 per plug & $15 per plug to put them in! I found that if you pull the battery & box and the air filter housing there is plenty of room: job takes less than 1/2 hour if you have the right extension.
It is like falling off a turnip truck to change the plugs in the 420SEL: are there some MB plug replacement horror stories out there?
p.s. just bought a set of Bosch Supers at Autozone for $0.99 each. Is that a deal, or what?
'91 420 SEL @ 199K, '92 SVX @ 181K, '93 SC400 @ 86K, '93 Kaw ZX-11 @ 30K, '87 F250 @ 181K , 2001 Valkyrie Interstate @ 6K, Y2K Honda NightHawk 250 with 1.5K, '88 420SEL I.@ 179K & the 2nd latest, an '88 420SEL II.@ 210K runnin' parts car, '85 F150 300/NP435