Yeh I've heard of adding a drop or 2 of dish detergent before, but never vegetable oil.
Volk oil will scorch in Sun, I've yet to purchase neem oil. I'm glad your alls experience with it
was better than the discontinued Volk oil, both horticultural oils. I wonder what makes one
more Sun tolerant than the other?
Conifers have strong cuticle wax, this makes their (actually pretty weak) foliage sun-proof. It's a basic chemistry rule that most organic* material can dissolve in other organic material. Some horticultural oils mix with cuticle wax, others dissolve and replace the cuticle wax. It depends a whole lot on the composition of the oil. If the oil replaces the cuticle wax, the foliage is somewhat less protected. If the oil doesn't have the same refraction (bending of light) or is more transparent than the cuticle wax, then the cells beneath it are less protected and prone to sun damage. They dry out faster, the sun has a stronger effect and the oil might even affect respiration (breathing).
Neem oil - especially hot pressed - is pretty waxy by itself; if you leave it on a shelf for 3 months, it's going to harden to a waxy substance with a layer of oil on the bottom. We need to heat it a little and sometimes use an emulsifier to get it fluid again. This doesn't happen with olive oil for instance. This is because of some harder to grasp chemistry; carbon chain length, double bonds and carboxyl groups.
Some trees are fine with having their cuticle wax replaced or dissolved, they just rebuild it on the go or their cells are more protected by nature. Conifers can re-secrete cuticle wax, but it's a slow process and this can take some time. When that layer of wax is disturbed too much, sun damage can occur because the plant doesn't have enough time to protect itself again. For this reason, both the emulsifier (stuff that binds organic material AND water, which usually repel each other; they need a helping hand, the emulsifier like dishwashing detergent) as well as the oil or wax need to be used in moderation. A little too much of either, or both, screws up the protective properties of the remaining cuticle wax.
In general I think it's easy to remember it like this: the more fluid/liquid a material is at room temperature, the easier it can dissolve cuticle wax. If it contains aromatic molecules (lavender, lemon, lime, paint thinner, gasoline), then it might be even more dissolving and more disruptive. The closer a material is to cuticle wax, the less damage it does, but the harder it is to get into a watery solution. I think anyone who has ever stored neem oil, remembers the gunk on the inside of their spray bottles. Heat that gunk with some warm/hot water, and it can re-dissolve again. That's actually the good stuff.
I hope this makes some sense. Carbon chemistry isn't my favorite part of being a biochemist, actually, it's the worst part if you ask me. But I hope that this explanation makes some sense without diving too deep into scientific terms.
*Organic in the sense of organic chemistry: carbon chemistry. Oil dissolves in oil, wax dissolves in oil, oil dissolves in wax, in a general sense everything that's 'organic' can dissolve in 'organic solvents'. Test this at home with a balloon and some citrus fruits; the wax from citrus fruits contains large amounts of limonene, limonene (organic solvent) dissolves rubber (a carbon polymer). You can pop a balloon with the stuff that sprays out of a lemon peel. Try the same with your average hair wax, and you'll just get a more shiny balloon.