I just purchased a used 2004 RX-8 from the dealership, but no service sheets came with it. Thus, I am not sure what type of engine oil is in the car. After much searching, I know that I should probably get a bottle of oil to top off the car once in a while. I will be driving from Las Vegas, to South Carolina. I won't be in freezing cold weather.

Without knowing what's in there, is it safe to assume that I can top it off with a 5w 20 non-synthetic oil with no problems or do I need to find out exactly what type/brand of oil is in there? I'm guessing I could just buy a bottle of Castrol GTX 5w-20 and be ok?

Don't worry about it. Put in 5w20, your choice of brands. This small of an amount won't matter, even being a different weight isn't a problem.

I use 5w30

but I guess u should do an oil change before u go for a long trip.

I just purchased a used 2004 RX-8 from the dealership, but no service sheets came with it. Thus, I am not sure what type of engine oil is in the car.

More importantly, you don't know how long it's been in the car. Do an oil change with whatever you plan to keep using.

Good info ^^^

Any 5w-20 (oem spec) or 5w -30 will do, but personally I would do a complete oil/tranny/diff change.

I just purchased a used 2004 RX-8 from the dealership, but no service sheets came with it. Thus, I am not sure what type of engine oil is in the car. After much searching, I know that I should probably get a bottle of oil to top off the car once in a while. I will be driving from Las Vegas, to South Carolina. I won't be in freezing cold weather.

Without knowing what's in there, is it safe to assume that I can top it off with a 5w 20 non-synthetic oil with no problems or do I need to find out exactly what type/brand of oil is in there? I'm guessing I could just buy a bottle of Castrol GTX 5w-20 and be ok?

Castrol GTX 5w-20 - For topping off on a long trip, you are fine.
There are the weight debates, the synthetic debates, but as long as you are reasonably confident that the oil had been changed sometime recently, you are ok.
I would get a change before a long trip, and everyone should. But nothing wrong with the spec oil in moderate conditions for a top off.

I would get it changed as well. It's just a lot safer that way.