Yes it takes 11 liters to fill up the cooling system and I would definitely go for a anti corrosion type of fluid. But I guess most of them have anti corrosive properties.
There are also additives that supposedly make the engine run a bit cooler. Very interesting but I haven't tried them yet.
Motor oil is always a topic of much debate. There are so many views and opinions.
On one end of the spectrum are the people that claim you never need to change the oil only the filter and then top it of. On the other end are the people that only want the best of the best and believe every new add from the motor companies. I think you should aim for something in the middle.
I personally always buys a 60 liter oil drum 15w40 semi synthetic and change every year of after about 8000 kilometers. The make doesn't bother me and this way I pay only a few euro per liter. I do believe that oils of today are a bit better then the once available of the early 80 so I think that will allow me a bit more millage between changes. I would not use a fully synthetic oil because I've been told that they could damage the oil seals. Don't no if this is true but I don't want to risk it or play the big bugs for those high priced designer oils. There are however people driving there Murena's with these oils. If you just rebuild your motor with all new oils seals it would make sense using a fully synthetic.
I just replaced the transmission on my car and bought a 75w80 GL5 oil. (Unknown brand) It's a very thin oil almost water like it is definitely a lot thinner then the previous oil. And I now have a leaking gearbox even though I replaced all the oil seals. But this might not have to do with the oil I could have damaged a seal or used insufficient liquid gasket to close the differential housing. Anyhow I'm disappointed and will have to pull the transmission again
I would not bother with GL4 since this is just a lower spec oil. GL5 is replacing GL4.
You are right no to use dot5 I would only consider dot5 after rebuilding the entire brake and clutch system. This is something that I will probably be doing next year but I'm still not sure if I will use dot5.
Does anybody have any experience with dot5 on the Murena??