Copper fuel line sizing & fuel oil additives

Here is my home setup:



If you've seen my other thread recently (apologize for its length), you'll know currently my home fuel oil-burner system is suffering from nozzle "after-drip"...... that can be pinpointed to either incoming-suction copper line narrowing (from the fuel oil slowly gunking up the sides of the inner copper pipe over 20 yrs) and/or "air" getting in the lines from the tank to the Afriso filter. Or possibly both. My burner/boiler unit still runs good, but you can tell-----because of the -0.4 to -0.5 vacuum it is pulling on the suction line during a burner run----that something needs fixing or replaced, fairly soon. And that 'something' would be the copper lines coming into the house.

So, one way or another, I am going to be laying new sheathed copper pipe (hopefully when the weather turns warmer here in 3-4 months) or, the dreaded option, maybe now. There's also the option of installing the new Afriso Flocop Tigerloop device I bought, but that means messing with the current 20+ year old sheathed copper 2-lines currently coming into the house. Basically, if I do this, I'd be going from my current 2 line setup to a one-line, and (I've no room to screw anything up as there's no service loop and/or extra copper length to play with. This is true for both the suction line coming in, and/or if I decide to use the return line as my "new" suction line).

Ok, my questions:

1) My current suction/return copper lines are 12 mm (near 1/2"). When I lay new sheathed copper, given the distances in my pic from burner to undergound tank, and the 3-4' lift, would you guys go with the same diameter tubing, or would you go down to 10 mm (3/8") or possibly down to 8 mm (1/4")?? I always thought, to a certain extent, that having a bigger diameter pipe was a good thing. But what confuses me is the new Afriso TL device (and all devices I researched) provide only compression fittings*** for incoming-suction line sizes of 10mm (3/8") and 8mm (1/4"). What happened to 12mm or 1/2" tubing? Is it now consider not a good thing?? My current old Afriso fuel filter on the side of the burner/boiler (see pic) is 12mm, so....?

***Yes, I realize the affinity for "flared" fittings in North America, but in Europe, they have developed 'different' compression fittings (pic of them in other thread) that are considered better, longer lasting and idiot (installing) proof. And those are what is spec'd here, for home heating installs.

2) Fuel oil additives - do they help or not? I realize this might be voodoo science and cause serious disagreements, but does anyone here regularly (say at every fill up or once a year) add an additive to their tank? I have easy access to my in-ground tank, so adding something is no problem. I had always been under the impression that in today's day and age, fuel oil & any gasoline-type have already added to them various additives---especially in the over-regulated European (German-French) markets. Still, would adding something to my 800 gal tank help at every fuel up, or maybe help in the future to keep my copper lines not-so-gunked up over time (like is happening now)?? One old guy I met in my neighborhood, he dumps about a pint of "white vinegar" once a year to his underground fuel oil tank. I was like: WHAT? You dump VINEGAR in there?!!


Thank you for any advice, hint and/or tips.