Yes that is a fuel tank underneath, but I don't think it's Ohlsson supplied one. Some of these engines had a plastic fuel bowl under the carb needle valve (use of the wrong fuel will destroy these) and others had a metal tank that fitted behind the crankcase.
The Octura information I have mentions glow fuel containing castor oil. I can't really advise on the ratio as it would depend on the performance required. The introduction of in glow fuels caused some problems with Ohlsson engines and could cause catastrophic failure i.e. the con-rod bending in early engines, they changed to using steel con-rods which would then cause the head to be blown off instead.
If you interested in the history of the early Ohlsson model engines and the eventual problems when glow fuel was introduced, then the reprinted articles from 1966 in the 1999 Thermaleer newsletters are worth reading, see pages 12 to 13 of issue #59, pages 12 to 13 of issue #60 and pages 16 to 17 of issue #61, available as free downloads from here: http://www.sam600.com/old-site/b_thermaleer.htm
David