I know that dark liquids gunk up
coils. Are top coil or bottom coil best to use with liquids like that?
If you're referring to common production coil/head assemblies like Kanger, Aspire, Innokin, etc - and I assume you are, there's not going to be any difference top, bottom, dual or single - although the lower resistance
coils might run marginally hotter. None however produce sufficient heat to offer any serviceable level of self-cleaning, if there are enough products in the juice to effectively caramelize on the coil/wick.
You can (after removing rubber parts) torch them to carbonize (and blow or rinse off) contamination... the standard silica wicks can take a good bit of heat, although you might fry a coil in the process, if you aren't attentive.
As commented on by Topacka,
frequent dry-burns/clean-offs would be an option - but let it go too long, and without a torch... you're plucking an new coil/head from the package. ;-)