Hi.
Should I be adding some additve for the unleaded fuel?

Simple answer: Nope. There is no quality or compound in leaded gas that these engines or systems need in any form.

__________________
Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

Thanks Tom.
Previous owner told me that but wanted to double check. K.

Have i heard correctly, that some people add ATF (auto box oil) into the fuel?
What mix/ratio?

This would be to clean the carbon off the valves. The theory is ATF burns hotter than gas and just makes the carbon build up crumble away.

You can do the same thing with some fuel injector cleaner.

And when you put ATF in the gas you can expect massive amounts of exhaust smoke until it burns away. We used to, in the 60's, pour it through the carb in order to decarbonize the valves. It looked like a smoke screen from a Patton tank being laid down.

I you run your car on leadfree .And its made to run on leadfree fuel , then it ok .As old cars needed lead to stop damage to components in the fuel system and engine . Thats why valve seats and valves were replaced with harder ones.All classic cars over here in uk have had this mod done.

Things you can/should add to your fuel tank:

• Fresh gasoline (Petrol for you in the U.K.) with a minimum AKI (Anti Knock Index) of 91. My memory tells me that's 96 RON equivalent, but you can use Dr Google or similar to check... I believe that's what you mean by 116, the 3.5 m116 will require higher grade fuel to prevent engine knock.
• A can of Techron fuel injection system cleaner every 6-12 months.

Anything else is putting your money up in smoke, and/or potentially damaging to the engine, injectors, fuel pump or other components. If your engine is not running right - which may be why you're asking these questions - you should look into troubleshooting your ignition system and fuel injection system.

__________________
Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

Actually, since you posted about it being a South Africa car in your other thread:

Are you sure you have a 3.5 M116? Aside from reading the engine plate, which may not be the easiest, you can usually tell by the yellow injectors. But, those may have been replaced too.

Honestly, you may want to contact MB & have your chassis code run to see what engine & its compression is. If delivered for South Africa it may either have a low compression 4.5 (7.0-8.0) or low compression 3.5. That would mean you could put in a much lower grade of petrol.

__________________
Current:
2021 Charger Scat Pack Widebody "Sinabee"
2018 Durango R/T

Previous:
1972 280SE 4.5
2014 Jeep Grand Cherokee Limited "Hefe", 1992 Jeep Cherokee Laredo "Jeepy", 2006 Charger R/T "Hemi"
1999 Chrysler 300M - RIP @ 221k

Idle.....lol....l remember adding some GM carbon cleaner into my 1969 Continental way back when. You add it slowly, then a lot at the end to make the engine stall. When I restarted it, the smoke was soooo thick that the fire department show up!! Thanks for the memory!!

__________________
All the best,

J. P. Mose
1968 250SL
1970 280SE 3.5 Cabrio
1987 560SL
1990 560SEL

Don't use ATF , it isn't designed to be burnt.

For carbon removal, Techron is the only thing I'd use maybe once or every other year.

For engines in general, anything with an aluminum head has hardened valve seats making a lead additive redundant.

I stick a couple of ounces in of marvel mystery oil in my tank. Car seems to run OK with it and I have friends who swear up and down that it works. I can't really say that it does or does not but it does not seem to hurt it and I suspect that it adds a little bit of lubrication.

__________________
With best regards

Al

He has a 3.5. Outside the US, the 4.5's were not sold in W108 / W109 body styles. Certainly not in the UK. I was in London last week and saw some neat old cars on the road. It was a nice surprise as I thought thought the government would have legislated them out of existence.

__________________
With best regards

Al

Trunk says 3.5. Lol.


@Alabbisi......
I'm sure that day will come but not yet.
Hence me finally getting much long wanted V8.
In the UK pre 1976 cars are road tax free.
So nice to just pay £45 for MOT (roadworthiness test) and £85 classic car insurance per year.