Terminology. Full detergent...not rally any such thing. Some oils are know to be more detergent than others - meaning they clean better.
All oils except NON Detergent like ND30 are detergent and haven't been used in cars since around 1953. They are only suitable for reciprocation oil style shop air compressors or oiling a fan motor or bearing and then should be a 10 or 20 wt.
People have a weird love/hate/trust relationship and opinions about oil with only a VERY SMALL personal amount of experience to base ALL these opinion they have that they often claim as fact or the way it will be in ALL applications.
All in all it make not that much or a difference.
For all the people that say "I only use full syth brand ***, and ALL my engines have gone to over 225k" there are just as many people that have never use a synthetic and their engines go 225K also.
Most engine failure are not oil type related or even oil weigh related. Most are not even oil related. On the lack of lubrication failures...oil pumps can go out but most are neglect and lack of oil changes and lack of ENOUGH oil!
So regularly changing the oil with any decent quality or brand oil is more important than anything else....and checking to keep the level up.
When an engine is run almost 2 qts low on oil, the remaining oil that is in there just gets overheated and coked up SO much more than if the correct amount was in there.
Snowball effect.
Synthetics, all that I know of have better detergent or cleaning or keeping engines clean inside abilities than conventional oil. This is also why you can go longer between changes with syn...but that's not why all people use it.
They did have a lot more leakage issues with synthetics a long time ago.
Probably because or multiple things. Worse manufacturing tolerances, worse gaskets (cork), less RTV and sealers used. Could have made a difference if a new engine started it's life on syn to the gaskets vs reg oil first. Lot's of variables, but it wasn't a myth back then.
There's a lot of things in oil and the seal sweller stuff might be not the best name for it but they do tend to negate these issues in most cases.
I have had cars switched to syn leak or seep some at first but then go back to not. I have had some not stop. Have had some stop by going back to conventional.
I had a 245k 5.4 that was dry as a bone only start leaking at pass rear valve cover after cheap Rural King (Citgo at the time) full syn and a bottle of blue STP.
Had used Mobil 1 5w-30 for years and no leaks....Was it the STP? the cheap oil with no seal swellers etc? or just it's time to leak at almost 250K.
We'll never know and I haven't changed it again to play with it to see if it slows down. It's not terrible the way it is.
There is always a change with an excessively high mile engine that full syn can be used or leak at a decent rate that would be more than a conventional.
The weight also has a lot to do with it.
I feel synthetics are for lower mile engines or severe duty racing or long hours and idling where you want to try to GET to high miles.
You are already beyond high miles so it is about damage control.....or symptom control to say it nicely. Wan't to find what will slow down oil consumption/leaking without "motor honey-ing" up your engine to where it can't flow back down the return holes to get back to the oil pan and pump.
Some people will freak when you talk about anything but 5w-20 in a 4.6. Funny though they came from factory for decades calling for 5w-30 and they some freaked about putting 10w-30 in them. Maybe some people just like to freak or maybe they are freaks....lol.
You can run about any commonly available oil in them at temps above 50-55F.
You could even run 20w-50 but even I wouldn't recommend that as most are conventional at this weight and if you've ever put qts or oil in the freezer for days and poured them....you don't want this, or lots of STP ot Lucas etc in your engine when the temps get down to 40 or lower.
I ran 15w-50 full syn mobile as if flows A LOT better than 20w-50 conv even though it sounds like it would be close to same.
We ran two 4.6 town car limos on this for years in Ohio winters. Not inside kept cars so cold starts. One was 92 with 121K the other was 94 or 95 with 180K but Jasper reman engine in it. have no idea the mileage but it had smoking valve stem seals on take off after idling so it has a few or maybe someone reused old heads on jasper shortblock if they sell that way.
It slowed down consumption and didn't smoke as much or smell as bad.
On a higher mile 2v ford modular engine, at temps above 50 I would use no thinner than 10w-30 High mileage, and am fine with 15w-50 syn too.
Not a fan or 10w-40 but it is thicker than 10w-30 just keep an eye on it and don't let it get too black looking.
You could use 30HD or also called SAE30 also. Thicker than 10w-30 and fine unless its going to be under freezing out.