Use the viscosity as the owner manual states.

The API motor oil service rating has changed since your 240 owner manual publication. Motor oils perform better whether mineral (dino) or synthetic (made from dino oil).

I use Mobile 1 10W40 in all three.

The PCV never clogs. With mineral oil on my K-Jet 1970s 240s, I used Castrol 10W40 mineral oil and the flame arrestor would become clogged. And I put miles on all of them. No stop and go and stop without allowing the engine to warm allowing the oil to fully heat and the condensation and such to steam away.

Why a motor flush? Sludge is sort of a myth unless you do not change oil and drive go and stop and go without allowing the engine to fully heat, along with the engine oil, and also the engine control is out of tune, usually burning a way to rich fuel to air ratio, or other issues preventing proper combustion resulting in a 'clean' burning engine with low emissions before the catalytic converter.

Too much ZDDP does not help the catalytic converter. Thank you for making mention. We're not motoring about in solid tappet flat head, or side valve engine from the 1930s-1950s. Nor are we motoring on over massive primitive V8 engines either.

Mr. Yount and his Volvo 282 gets better fuel economy with probably better emissions than a typically tuned stock 240. Probably as good as a properly tuned 240 with LH-Jetronic / EZK as you and I have.

Good job with the engine oil seals. Your PCV is not clogged, then? Breathes freely does it? I've posted a method to verify PCV breathing and seal. The glove test has it's use to loosely test for combustion blow.

I was able to get up to 26 MPG and 30+ MPG rarely in my K-Jetronic 240s. Around 1% CO before the cat. I do not floor the gas pedal in stock 240 though.
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