XXX3!!! ^^^^
No additives and quit using the synthetic for topping off.
Topping off due to oil consumption?
Besides using PCV thingy 12572717, and buying / DIYing a decent catch can, I can only think of the following:
For topping off, any 5W30'll do; it'll either be consumed before or changed out by the next oil change.
If your powertrain works or plays HARD, change the oil more often, and probably install some kind of oil cooler.
(The motor oil portion holding the crank up off the main bearings can easily reach 257F, hotter if working harder.
Some motor oils are already 'smoking' @ 248F. Smoke that does not cool off and reaches the PCV is consumed.)
(Are taller valve covers available for our Gen3 engines - kinda like the taller valve covers for the new PPV's?)
Not even the ATF in the oil or fuel?
ATF is for transmissions. Only ATF additives I use are made by LubeGard.
Engine oil is for engines. Managed to avoid engine oil additives my entire life.
That said, bobistheoilguy.com recommends a few motor oils specifically for slow gradual gentle engine cleaning.
Haven't tried any myself. Maybe someday, maybe not ever - need convincing.
Fuel is for fuel tanks.
Besides Chevron with TechRon (albeit at DOUBLE strength), or Marvel Mystery Oil, doubt I'll try anything else.
As for coolant, I ALWAYS add RMI-25, then flush it out every two years, and add RMI-25 again.