Hey guys, I worked for Fuchs lubricants (Pronounced Foooks) not the other way!! as their Regional sales and production manager for several years so have a little bit of knowledge as far as oils are concerned. Heres my 2 cents worth on the subject.
As far as oils in crank cases are concerned, changing it at the start of the season is a very sensible idea. Firstly it is cheaper than wearing some piston rings or valve seats out (which can happen with old unused oil). The water that may accumulate will disperse - evaporate - but will deplete the additives, so this also makes for a good idea to change out the oil.
The use of Mobil one is a bit over the top. It is expensive and unless you are running any motor (including car engines) at WOT all the time you just dont need it. Valvoline 20w/50 is fine for warmer climates - but look at getting something a bit lighter in viscosity for cold climates, if you can get a good 10w/40 or there abouts it would be even better. The lower the first number in the viscosity range the better, as this is the cold viscosity of the oil. So on start up the oil is very viscous and will lubricate faster. The higher the second number the "thicker" the oil is when hot. This means the oil will hang in there longer when hot.
This is all laymens terms, but just stick with good quality well known brand oils and you will be alright. As far as the transmissions are concerned always stick with the manufacturers recommendations, the oils will be identical no matter what company you use.
Cheers, Tony

That is great info Tony! I'll assume chemicals react the same down under! I think I'll forego the mobil one and go back to good ol dino oil and change it fall and spring!

Yeah Base oils are the same in Oz as in the Northern Hemisphere.:)
The additives are also basically the same, there really are only 5 to 8 major additive manufacturers around the world (which makes lubricant manufacturing easier). Because of this, most oils will be basically the same as each other. Each company has different compositions to each product (engine oils anyway). This is what makes each of them different.
If you find your engine (car or boat) is using a little bit of oil, you can safely change the viscosity to reduce or even stop this problem. If you are using a 15w/40 and you use a little bit of oil, change to a 15w/50 (increasing the top number - viscosity) will do this. Remember the first number indicates the "cold viscosity". You wont be using oil when the engine is cold.
If anyone is using 2 stroke outboards, ALWAYS go for a big brand 2 stroke oil.
Dont go cheap it is not worth it. I have seen tons of motors laid up simply because they chose the $5 bottle over the $10 one. There are heaps of companies producing these, and with this particular oil there are many variations.
Crank case oils are basically the same. There really is only one or two ways of making it. The base oils are the same and the additives also are very similar. Just go with the manufacturers recommendations, it will be fine. Dont ever put a heavier oil in than what they stipulate though.
If anyone has any queries just ask - boat or motor vehicle no problem.
CHeers Tony