Oil tumbled to the lowest levels since early January as long-term headwinds overwhelmed the positive sentiment from a strong US jobs report. Brent crude, the global benchmark, dropped below $80 a barrel, while West Texas Intermediate slid to less than $74 a barrel. Futures for both grades climbed earlier in the session as record-low US unemployment figures spurred optimism that demand would hold up. But those gains evaporated as concerns about swelling US stockpiles and weaker-than-expected C ....