...seems your question implies another: could the use of 20W50 oil in the tropics be detrimental...as opposed to straight-viscosity such as 40-wt or 50-wt?
In more than a few instances what the service provider will install is what it will have decided in its own economic interest to keep on hand, so the question could be: why did the tropically situated service department not keep or have in stock, say, the straight 40-wt or 50-wt?
Well, most likely it is more economical to stock fewer as opposed to greater numbers of oil varieties; and so, if it comes to a viscosity choice between single-weight and WinterSummer grades, the latter is preferable for two reasons:
- Because most new car manufacturers do NOT recommend single grade oils
because cars must operate in tropical as well as temperate conditions, and
- Because lubricating with (in your case) a 20Winter50(summer) grade oil where winter never occurs is tantamount, in practical terms, to lubricating with a 50wt oil in ambient conditions never varying from such, in tropical climes, as typically requires or is compatible with 50-wt oil.
As for your own servicing, following the specified oil recommendation where available will do not harm.