Bearing lubrication

Strange noise from the main shaft supporting bearing in an another air compressor obliged me to maintain the equipment last late at night. The production date is 1978, so over 35 years have passed, I wonder why the lubricant inside of the main seal bearing has been lasted till now.
According to the information from bearing producing factory, lubricant grease should be replaced 3 to 4 times a year if used in the condition of below 7,000 rpm at 80 degree celcius, so it's unnatural to have kept rotation till now after 35 years of production. I found it possible to be continued without replacing bearing by just applying lubricant by splay grease. Strange noise disappeared after that. These equipments would last me long from now as consumer durables.