A few points here
1) j-mans oil wont affect the sand because:
"The Trollkages oil, that Gaara has mentioned to seep through his sand and destroy it, isn't exactly just oil. It is a mixture of water and oil. This is proven when it was referred to as a "
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" thus implying that it isn't just oil, but water as well. This is confirmed to be the case, when Onoki was explaining the mechanics of Joki Boy. He mentioned that the Mizukages body contains
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.
[*]Normal Oil
Now, the substance that others like j-man and his toads use is pure oil. A
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and very flammable substance.
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[*]
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The difference between the two, is that the oily water the Mizukage uses is very slick and slippery. Reason why Gaara noted it to seep threw his sand. While in contact with the sand, the watery oil turned it into a gooey, slimy,
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. Normal oil lacks the watery substance to make is slick and slippery in order for it seep threw Gaaras sand as effectively. It would just cause the sand to stick together and clump up. Also, it is worth mentioning that oil and water can not mix. So bringing up Suiton Oil attacks is futile because it wouldn't work. The oily water is innate only to the Trollkage."
2)making sand is practically of no effort for gaara even as a kid.
3)gaara can just use a sand hail sand arms combo where bunta is attacked from above while the sand arms can be made beneath bunta,making itimpossible to hit them before they crush his legs