If the car has been neglected with thick carbon residue coating all engine internals after excessively long oil change intervals then it could be worth doing, but at a risk of dislodging clumps then likely to block the sump filter.

Could be worth investing a tenner or so in an endoscope, linking it to phone or laptop and have a good poke nose inside the rocker cover and inside the sump while the oil is out, see for yourself what's what.

I did it once to an 1800 Marina, using Comma flushing oil. This didn't seem to get very dirty and it probably wasnt worth doing, especially as there wasnt, IIRC, any particular issue with the engine.

It was the first car I'd done much work on and I was a bit keen.

I'd think the plugged hydraulic tappet mentioned below might be a special case where it might have benefit.

More recently, I found a 50:50 blend of Mobil Delvac MX15W40 and China Petroleum Corporation straight SAE40, not as a flush, but for a 6 year oil change interval, to be very effective at removing varnish.

The Delvac truck oil will probably have a high dose of detergents, The SAE40 is likely to be Group I, which is known to have polar content giving greater solvency, and it will also raise the viscosity and shear resistance a bit. Mix was probably about 20W40.

Probably wasnt necessary or desirable to leave it in 6 years, and certainly wouldn't be on a diesel, but it didn't seem to do much harm to this 1986 engine.

bobistheoilguy.com/forums/threads/engine-after-sit.../

You dont really need an endoscope to look under the rocker cover, Main application would be looking inside the cylinders.

I just got one to help me treat an ear infection, when I no longer have a car. Oh, the irony.

The Delvac truck oil will probably have a high dose of detergents, The SAE40 is likely to be Group I, which is known to have polar content giving greater solvency, and it will also raise the viscosity and shear resistance a bit. Mix was probably about 20W40.

You dont really need an endoscope to look under the rocker cover, Main application would be looking inside the cylinders.

IIRC the OP's car is a modern Diesel, many of which are prone to injector seals leaking after some time, this causes serious carbon sludge and clumps to build up around the injector seat due to blow-by, which contaminates the oil and clogs the oil pick up strainer directly resulting in engine failure.

You can't just whip the rocker cover off on these engines any more, injector pipes have to come off (single use only in most cases), to inspect inside the cylinders on a Diesel means removing injectors too.

Interesting point about good commercial Diesel specific engine oil and its cleaning properties,

Re not being able to easily remove the rocker cover...

Ah...progress. How deprived of it I was, with only two bolts and an air filter to remove.

SOOO DEPRIVED!

Re Diesel specific oil, the Delvac MX was a dual use "mixed fleet" oil, SJ/CH IIRC . I do have quite a lot of diesel specific (ie diesel only) CPC oil (CF IIRC) and would probably have eventually used it in the car, had that survived

I have seen "somewhere" that this is a bad idea, but don't know what, if any, technical basis this has. I would probably have continued to use it in a blend with the SJ SAE40 but dont of course know if this would have mitigated the alleged problem, since I dont know what it was.

I'd used the MX on its own fpreviously for maybe a couple of years and 3 oil changes, because there was metallic sludge in the oil, addressed by relatively short oil change intervals, and removing the sump (which didn;t drain very well due to a lip around the drain port) a couple of times.

Although I think this is probably a good oil with a healthy dose of detergents, it did not shift the varnish on its own. I THINK the varnish removal was due to the Group I polar solvency of the SAE40 when I used this in the mixture. This is a known effect. Power stations had a lot of turbine oil varnish issues when suppliers "upgraded" to Group II and III oils.

No sign of oil degradation in the oil over the 6 years and engine very clean, but chromatograms showed a debris ring, which might have contributed a bit of excess wear, and it was pretty black.

Still have quite a lot of oil. Probably take quite a while to use it up in my (broken) motorcycles.

These are of course all mineral oils. Since they were lasting at least 6 years on my mileage, there was absolutely no reason to use synthetic, and its quite hard to find a documented technical reason to use synthetic in anything unless you are REALLY stretching the miles on your oil change, which I probably wouldn't do.

Edited by edlithgow on 11/10/2023 at 00:35

Unless you have an engine that is prone to clogging up (e.g the eatly 2000s Chrysler 2.7 and a certain MY Toyota v6) then it should not be needed unless oil changes have been missed. Trouble is once it’s needed it’s too late and becomes dangerous to flush! These products should really be used as a preventative measure before the engine becomes clogged in the first place, if at all.

You mean like taking Penicillin, just in case you get an infection?

Your suggestion sounds like an Oxym*o*r*o*n, wrapped in brackets!

No, not like that. Think of it as having treatment in the very early stages of the disease, rather than waiting for it to take over the entire engine and then nuking it.

Engines don't get clogged up overnight unless there's a serious issue elsewhere. The buildup would be gradual (if at all). By doing a flush every year, theoretically you clean out the very small amount of build up, before it gets to the stage where it could become dangerous to do a flush that would dislodge large enough chunks to block oil feeds.

Thank you all! Some great replies and interesting thoughts as usual.

If we're talking about taking preventative measures on a diesel engine, one way would be to give your car a few gallons of V-power (super) diesel every few months. My current car seems to like it and it costs about the same as adding shots of conditioner every so often. I've never seen the need for an engine flush in 60 years driving.