I've also looked into this recently.
There are 2 answers:
- Official and safest - Use BMW or a guaranteed compatible alternative like Comma G48 (Halfords)
- Unofficial and less safe - Just take the advice of those who have tried other, we'll say, 'broadly' compatible brands/types and suffered no ill affects based on several years of use. Note any damage is likely to come from long term use only so it might be some years before you can be truly happy with it.
BMW seem to have a fairly unique spec which for whatever reason is not produced by third parties in the same way that LL-04 oil is.
For what it's worth, I don't think you'll find BMW antifreeze is that expensive. It's about £9.50 for 1.5L which seems to be comparable or cheaper than most big name products. Certainly it's cheaper than the Comma G48 in Halfords. That's £17 for 2 litres. So that's £6.33 and £8.50 per litre respectively. The Halfords is still £6.37 per litre with a trade card!
Anyway, I need some for 2 different cars and it will be BMW. I'm not wasting any more time trying to save a few quid by trying to find something that might be safe to use.
Equally important is that you use distilled water to mix it with not tap water. And that can spark a debate too but at £1.50 for 2.5L it's not worth taking the chance. Also stick to 50:50 or slightly weaker mix as too strong is not good either.
Set the heater controls to warm/low fan to help discharge the system. And drain the engine block too.
Even then you will be left with several litres in the heater core you won't be able to get out. And who knows what antifreeze was used by the previous owner. And what the strength of that mix is unless you check it before you start, ie what's left could affect the mix you think you are putting in.
And of course you don't really want to flush the system with a garden hose otherwise you will introduce tap water you'll never get out.
Then you'll want to change the thermostat and water pump if you don't know the history.
Simple question that ends up with a long answer! And the fact I doubt if even a BMW dealer/independent does this really properly - ie drain every last drop of the coolant, use distilled water to mix and use BMW antifreeze!
I think the safest approach for the home mechanic is drain what you can, BMW antifreeze and distilled water. If you don't know the history of the car change the coolant every year for the first 3 years so you get as much of the original stuff out over time from mixing in.
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