The best sunscreen is one that you are going to use. Any amount of sun exposure can change the pigmentation of your scar. It is also important that it is at least SPF 30 or higher. There is evidence that most people don't apply sunscreen thick enough to get the full SPF out of it (Photodermatol Photoimmunol Photomed. 2014 Apr-Jun;30(2-3):96-101). Therefore, higher SPF is always better (with this particular article recommending SPF 70-100). I'd recommend a sunscreen with physical blockers (zinc oxide or titanium dioxide) as they are more effective at blocking out the sun's harmful rays, although they do not rub in as well and frequently end up being pastey. If you end up using a chemical sunscreen, just be sure that it has UVA and UVB coverage.
Without seeing the lesion its difficult to determine what it actually is. I recommend seeing a dermatologist to have it evaluated. If it needs to be removed they will refer you appropriately.
