Here's some more info I found from looking around the web:
Old Engine Oil
Old Engine Oil is the second Harviestoun beer to win the competition of the British Nationwide Supermarket chain TESCO.
Old Engine Oil is a near-black brew with a silky-smooth rummy aroma, a coffee-ish palate, and a suggestion of the darkest chocolate. It is made from pale malt, roasted
barley and malted oats. Old Engine Oil is well-hopped with the sweetish variety Galena from Washington State, Worchester Fuggles and East Kent Goldings.
The PG (present gravity) of Old Engine Oil is unusually high. This is because Ken Brooker wanted to produce a beer that had the character of a barley wine, i.e. high PG, lots of mouthfeel and high viscosity but with only 6 % ABV, rather than the more normal barley wine ABV of about 8.5 %.
He has used a high mash temperature to give the required wort profile. The trick is to balance this sweetness with the hop bitterness and acrid flavors from the Roast Barley.