If you have rust deposits in your radiator then there is a 95% chance that the metal coolant pipes to the engine are corroded and are just waiting to leak. They also cause irregular temperature rise as coolant is not circulated at the required rate.
clean out the entire system with a flushing agent (usually citric acid) and inspect and replace the coolant pipes and other parts as necessary. Fill with correct concentration of coolant and distilled water to ensure proper cooling system working.
You should never leave a radiator that has been in service dry for more than 10 minutes - e.g. removal of radiator due to some repair. If you have to remove it - put a bung on the lower pipe flange and keep it full of water. (This was standard practice in my shop - the rubber stopper bungs were about Rs 5 or Rs 10/piece)
If you are installing a used radiator reverse powerflush it with water first then some very dilute conc of radiator flush filled for about 5 minutes and then flushed out with a garden water pipe. You will see some really nasty stuff come out of it.
for oils - your engine is specd for oils grades that are considered obsolete today, you can use them or something higher grade - as long as your engine temperature is under control and not too cold or too hot, using a good filter and a good working PCV system and good tune your engine would not raise issues of oil problems