1966 Mustang 289 Hot weather Engine Oil Advise & Suggestions

I have a 1966 Mustang C-code 289 with 82,000 original miles (recently purchase form the US),

I would like to know what type of engine oil would be recommend for my weather conditions. I already bough the filter Motorcraft FL-1A. I wanna know whats the best engine oil for hot weather, I was originally gonna purchase 10W-30 Motorcraft oil but the dealer in my country only carries the synthetic oil.

The weather is hot and humid here, the temp reaches 50 degrees Celsius (122 Fahrenheit) in the summer Lowest 5 degrees Celsius (41 Fahrenheit) for less than a month. I would say the average temperature is 35 degrees Celsius (95 Fahrenheit)

I did alot of research online and read alot of forum posts and websites, and I feel 10W-30 might not be right for my car, and believe 20w-50 would be suitable as in the cars original user manual 20w-40 is recommend but I cant seem to find that grade in my country or even online.

So my plan is to change the oil using the following material

Castrol GTX 20w-50 with ZDDPlus (a ZDDP Additive)
Motorcraft FL-1A Oil Filter

I dont want to switch to synthetic oil.

I need some advise as this is my first classic car, I am also planning to change the oil at 3,000miles intervals, please advise me on my choices.

Thanks for all your input.

Ordered some parts for my car from mustangunlimited and I got a flyer for Joe Gibbs Driven Oil, as im not in the US and this is not available in my country I wont be able to use it on my car, but I just wanted to share this information.

Joe Gibbs Driven Hot Rod Oil has Zinc,
Joe Gibbs Driven - Hot Rod Oils

and while searching the net I also came across another brand that has zinc its

Brad Penn, Penn Grade 1 Oil
Welcome!

just thought this might be useful for someone who might reads this thread later on.