Pretty much any oil type fluid will 'work' in your forks, that is to say it will provide some damping. As already mentioned weight is a matter of personal preference to some degree. Too thin, the fork will act like a pogo stick and keep bouncing. Too thick, the fork will be over damped and bumps will feel harsh through the handlebars, the tire may not stay in contact with the road.
I prefer my bikes to be more heavily sprung in the front with light damping. This makes them resistant to diving on hard braking but lets the fork react quickly over stutter bumps and the like.
Overall, fork oil is more suited to forks than motor oil. I doubt we are working these machines hard enough to really exceed the limits but engine oil is made for lubricating hot engines., fork oil is made for getting repeatedly squeezed back and forth through little holes. Go with fork oil for forks. I suspect that India being India and Bullets being Bullets they just spec'd something that anyone can get in any auto shop rather than some fancy oil. Remember, these bikes are the Model T of India.
It's good to replace the oil every year or two. It breaks down but it also tends to get sediment and gook in it from seals breaking down and metal rubbing on metal. While it's not undergoing the punishment of engine oil it does get worked. Replace it and keep your fork working.
Also, it seems from every fork thread on every website on every bike I've ever read, whenever someone takes their stock fork apart their are different volumes of oil in each leg (neither of which ever seems to the the recommended volume) and sometimes different colors and obviously different viscosities. It seems that getting the right amount of the right weight good quality oil in fork legs is not something any manufacturer takes too seriously. Change yours out just to be safe.
I think your 10W Silkolene is a good place to start. If it's too harsh, go with a lighter weight. Too bouncy, go heavier.
The other thing I'd recommend is to fill to a height rather than a volume. Tip each leg over and pump it until it's dry and all oil is out. Then fill with the recommended volume of oil and pump several times to get all the air bubbles out. Fully compress the leg and measure from the top of the tube to the oil. That's the oil level. Do the same for the second leg. No doubt, they'll be a little different. Average the two or pick on and fill both legs to that level, once again, make sure to pump several times to get rid of all the air bubbles.
A final word on overfilling, heavy oil, and excessive. Many bike come with soft front forks, especially Japanese bikes designed for 150# riders. They're just too soft for us heavier Americans. There are three cheap and easy ways to make the forks stop diving, and all three are usually the wrong thing to do.
Overfilling or sdding more oil: The air space you leave in the fork acts as a progressive, rising rate spring. If you fill the fork higher it gets stiffer faster during compression. A little of this can be good as it can stop the bike from bottoming out but too much is bad, it can raise the pressure in the forks to high levels and blow your seals. This is quite common in dirt bikes that have large air spaces. In road bikes more than 10-20mm means this is not your optimal solution.
Heavier oil: Many people with diving forks swap out the oil for a heavier than spec's weight oil This kinda stops the dive, or at least it dives slower so it's not so jarring, but as mentioned above the fork now reacts more harshly to bumps. This can result in a harsh ride, the tire leaving the road, and loss of control. Again, it's personal preference to some degree but if you stopped your fron end dive but your hands rattle loose over every bump you've gone too far and this is not your solution.
Preload: Another common remedy is adding preload. By using a longer spacer there is more pressure on the spring to start with. This makes the spring feel stiffer to a certain degree. Once more, a little can be good, too much can be bad. If you over load the spring you don't get full travel out of the fork and the spring is stiffer through it's more limited range. Once again, harsh ride and potential loss of control. It's better to have a stiffer spring to start with so that you get more travel and a better reacting fork.
The better solution to all of these issues is the right spring for your weight. If you go to any decent suspension shop they should be able to measure the rate of your current springs, take your weight, bike, and riding habits into account, and round up a more suitable spring for you. The difference is night and day AND allows you to get the right oil and oil height so the damping can work as designed rather than compensating for a weak spring.
This is the best concise description I've seen on the web about how to set up suspension right.
http://www.ohlins.com/Checkpoint-Ohlins/Setting-Up-Your-Bike/Underrubrik_1/Now with mild bikes like our humble Bullets our options may be more limited but even doing a few of the right things can make a huge difference in both comfort and performance.
I hope this doesn't come across as a rant, it's not. I just think suspension is something we all should understand more. Setting a bike up right makes it handle better in all circumstances and that keeps us all safer.
Hope this all helps.
Scott