good evening fellow forumers.

currently I'm driving an ACR50W Year 2006 Toyota Estima, clocked at 304k km
and I've been taken care of it (and also my father, whose was the previous driver)

I had issues that the engine oil starts to run low at 4.5k km after each oil change
- I will get the warning (I assume is low on engine oil - the text is in Japanese, and I don't write/read Japanese) whenever I make a sudden brake (assuming, the oil level fluctuation triggers the alarm)
- my mechanic told me for the long-term solution is for engine overhaul, and for the short-term solution is top-up the engine oil

my question is about the engine oil.
currently the engine (2AZ-FE) runs on Shell (I forgot the product name, but I know that it's fully synthetic) 10W-30
as for top-up - does it mandatory to use the same brand? or it is safe to use same fully synthetic (from reputable brands) 10W-30?

and as for reputable brands, I know Shell, Castrol. is there any recommendation? (I am planning on using this till the end of time)
before using Shell, it was running on Mannol 5W-30 (no issues so far. mechanic changed to Shell as the Shell that he had in stock that time)

recently I had the Water Pump replaced (with new one) as the previous one leaked.
mechanic top-up the coolant, but half-tank only.
but he instructed me to top-up with mineral water till the full-level mark
it's advisable to do that?
or need to fully fill (till the full-level mark) with specifically coolant only?

already solved.
water pump leaking.

will update the mechanic during next visit
as far as I remember, he didn't mention of sump