I too have an 8" skywatcher f5 with a 3x barlow i got off ebay for total $28, which is pretty average quality.
It's a let down, Jupiter at *300 is high contrast but poor resolution. But at least it looks sharper than at *150.
At 300x its hard to focus and annoying to track manually.
I just content myself with a 100x, 10mm plossel for planatery now.
Reconsider your need for a barlow. Instead perhaps go for a smaller eyepiece to give u perhaps 150-200x? Higher mags are rather overkill in most amateur scopes. U won't use it much.