Thanks for coming back to tell us the outcome, Paul. It sounds as if you have everything working well, and can expect it to keep on working for a long time to come (with an occasional blade replacement). I urge you to change the oil in your Honda engine, though: that is a lot less work than rebuilding the engine, which is what you will eventually be up for if you keep running it with dirty oil.
Referring to your initial post, the G100 has two dipsticks but both are for the same oil sump. It seems this is done because the engine is used for many different applications, and in some of these one or other dipstick may be difficult to reach. Here is the owner's manual for your engine:
http://engines.honda.com/parts/ownersmanuals/g100Note the odd way oil level is measured in all of these small Honda engines: you press the dipstick against the top of the thread but do not screw it in, when measuring. If you screw it in you will think the oil level is a lot higher than it actually is. Because you said the oil level is low and the oil is dirty, I am concerned that if you had screwed the dipstick in, you might actually have much less oil than you should. It sounds a bit like your engine may be running on the foul dregs at the bottom of the sump.
For the Australian climate Honda's recommendation includes a good quality, ordinary 20W-40 automotive engine oil. Make sure it conforms with an API standard for petrol engines, such as SG, SH or SJ. If the API designation begins with a C instead of an S the oil is intended for diesel engines. Note that the SG standard was introduced in 1989 and SJ in 1996. The second letter is moved onward when new standards for additives are developed. Where an oil is rated for both petrol and diesel standards it is suitable for both types of engine. Do not use oil rated only to an S standard (not a C standard) in a diesel engine: diesel service is considerably more severe.
