Wundershorts

Hi everyone

I tried going over the forum, archives and searches here but I could not seem to find the answers to my questions.
I am in the process of buying an '86 N/A RX-7 (FC) with about 123.000km on it (km - I live in Denmark)

My questions are:
- The car had some very distinct bubbling sounds from the cooling tube leading to the rad cap. Is this normal or does this indicate the dreaded "bubbling disease"?
The bubbling came after I had driven the car for about 15 mins and redlining it a few times, when I stopped the engine and checked the engine again.
At that time there was no water in the "return container" (if that is the word). I filled it with water and reving the engine made no bubbles in the container.

- When accelerating in 5th gear it made a whining noise - only in 5th gear. Does this indicate that the trans in worn (bearings, missing oil etc..)?

- The owner was using synthetic oil for the engine, would that be okay or should mineral be used?

In hope of some good and fast replys, thankyou in advance!