I was told that the forward oil creates the "pattern" i.e. defines the areas which you can play, etc. and the reverse provides bulk, allowing the apttern to hold up for longer.
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Reporting from England

Forward oil has the lane machine stop putting out oil, but the sponges that distribute it to the lane are still wet, so a buffed zone is created towards the pins, a gradual transition between wet and dry.

Forward oil avoids this buffed zone, since the oil is applied while the lane machine moves towards the foul line. Esp. when you have forward oil beginning in the back end area, you can expect a sharp wet/dry "border" which can cause some control trouble if you cannot play consistently.

Additionally, both forward and backwarsd oil tend to be applied at the same time, so the effects can overlap and are used to create certain defined oil areas on the lane.
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DizzyFugu - Reporting from Germany

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