Changing hydraulic/transmission oil and Starter sw

I drained out all the hydraulic/transmission oil from the 3 drain plugs (transmission, hydraulics, and rear diff) on my TEA 20 over the weekend, because I could tell that water had gotten into the oil, probably from sitting outside over the years. The oil was a very odd yellow in colour, very similar to the colour of egg yolk. I was wondering if anyone knows what this oil might have been originally.

I am changing it over to Duratran XL sold by Petro-Canada, because I want to use the tractor in the winter to do some snowblowing. I have a simple question as I do not yet have a Service Manual. Are all 3 - transmission, hydraulics, and rear diff - filled from the one location which is the large plug on the top of the transmission cover, next to the gearshift? I just wanted to make sure that I have not overlooked another filler plug.

I also have a question about the starter switch. Should the starter switch have some sort of interlock that prevents it from engaging once the engine is running? A few times I have inadvertently engaged the starter when moving from Reverse back to 1st?

Thanks,
Richard

Thanks to John, Paul, Jim and Mr Chapp for responding to my questions concerning hydraulic/transmission oil and the starter switch.

The reason that I have chosen to go with the Duratran is that I want to use the tractor in the winter so I will let you know if I have any problems. I do have a problem though and was wondering if anyone on this forum is also using Duratran as I find I am about 2 litres short. I purchased the 20 litre pail from Petro Canada and they say it is not available in smaller containers. See my post titled "Anyone using Duratran"

Thanks,
Richard