I'm almost positive your pipes are Cobras. Sound and appearance are their functions. All I've read suggest the Cobras cause a power loss unless the carburetors are re-jetted.
Valve adjustment is not hard. The maintenance schedule says Inspect every 12,000 miles. Most will say you can stretch that out considerably. Like 2, 3, or 4 times. You may find one or two slightly out of adjustment, but they are probably fine.
Oil, Oil Filter, and Oil Change Interval are very personal decisions. Do a search on the Tech Board and you'll see. As a starting point for perspective on oil and oil change interval, the maintenance schedule says every 8,000 miles on non-synthetic oil. Your other bits are probably fine, too, but if you'll come to a Wrench Party there are lots of folks who will climb all over it and offer their opinion. In the meantime, go to the Tech Board and Search.
The "velocity stacks" look great, but are non-functional. You will see some that have had LEDs put in them.
That's why we have Wrench Parties.
What Disco said, plus...
My opinion is that most people change their oil too often. You can easily go 10k with a quality synthetic oil. My interval has always been once a year or 10k miles, whichever comes first, using Mobile 1 15-50 synthetic.
The real potential for trouble lies in the rear drive line, but this is primarily due to poor maintenance by DIYers and dealers alike. It is imperative that the rear end be serviced at every tire change or around 10k miles and this service needs to include cleaning and lubing the pinion cup and drive shaft connection. The pinion joint is often ignored and the result is at best noisy and disconcerting and at worst leaves you stranded.
Add a periodic vacuum check of your gas petcock to your maintenance schedule and plan on rebuilding it occassionally.
There are Valks with 300k miles on them. 200k is not rare and 100k is common. A little bit of attention literally goes a long way.
Come to a wrench party and we'll work her over.
