I don't think I'd be happy about finding tar in my oil filter. Is that really a feature?
Since I have a summer car (2000 Mustang), I use Sta-Bil to keep the gas in the vehicle I'm not using from breaking down, since I can let one or the other sit for as long as 8 months. Aside from that, I let a decent maintenance procedure do what "mysterious" products only claim to do.

what was the oil like BEFORE you put the additive in?

I'm assuming this tar may have been sludge. Unless you were having a problem with your engine, you're sometimes better off to leave the sludge alone. Let it lie harmlessly in the bottom of the oil pan or anywhere else. Once you loosen it, there is no guarantee that it will all dissolve and drain. Could go somewhere else and clog oil lines scratch moving parts etc.
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