I think the solvent of choice would be "paint thinner" in the US which translates to "paint trimmer" in the UK I think. For the Fullers earth I would try to use non-clumping cat litter. It is an absorbent clay made from Bentonite like Fullers Earth, just not ground as finely.
An alternative would be to scrub with a dishwasher detergent or commercial cleaning containing high levels of tri-sodium phosphate (TSP) and absorb with cat litter. Here in the US, auto supply stores sell the cat litter material as "Speedi-Dry" same stuff but unscented, bigger bag and no cute kittens on the label.
They may not be the same shape on the other side. The top will have a chamfered edge, the bottom may not.
I should warn you about the poultice - 9 out of 10 stonemasons end up having to replace the stone because the poultice was ineffective or because the stain reappeared weeks afterwards. It depends on how long the oil has been allowed to soak in and whether you've applied degreasers that have pushed the oil further in.
There is a simpler solution - spread oil on the remainder of your drive so that it all looks the same.