What is the best chain and gear lubricant used by pro cyclists either for maintenance or for racing and how often is it recommended to apply? I have limited knowledge and so I have always simply used a product named Castrol EP 90.
That Castrol EP 90 has a gear oil viscosity rating of SAE-90? That is way to viscous for use on bicycle chain. Dirt & road grime is going to stick to it like crazy, wear out the chain & cause a lot of friction.
You need to use a bicycle specific chain lube. Which one depends on your riding conditions.
There are generally "dry" lubes & "wet" lubes.
Dry lubes are usually a solvent carrying a wax based lubricant. The solvent evaporates after application, leaving the dry wax in place.
Wet lubes are oil based without a solvent that evaporates away, thus remaining wet.
In addition, wet and dry lubes, as their names imply are best suited to wet & dry conditions. Dry lubes do not attract as much dirt as wet lubes, but wash off quickly in wet conditions. Wet lubes hold up to water much better.
Depending on conditions, a chain should generally be wiped down / cleaned ~100 miles or so & re-lubricated.
Specific product recommendations are off-topic here because they tend to become obsolete quickly. See What topics can I ask about here? for details.
Any bicycle chain specific lube product available from a local bike store or online retailer will be fine. What is more important is keeping the chain and drivetrain clean and lubricated properly. There are many online guides and YouTube videos you can consult about that.
A lube that is not specifically for bicycle chains like the product you have been using is probably not optimum, but not disastrous either.