Can I use my old auto thyristor 550FD on my new Nikon D90 DSLR

Good morning. You have likely already gotten an answer to this, but yes, it will.

Unlike less expensive models such as the D40, D60, D3000 and D5000, the D90 has a built in autofocus motor, which means that all Nikon F-mount autofocus-lenses (except two for the rare Nikon F3AF) can be used in autofocus mode for all newer generation autofocus lenses like the Tamron 200-800. However, you may want to look around a bit, as Tamron has come out with a new 150-600 lens that is simply terrific (I know, I own one) and the Big Sig from Sigma is equally great.

FYI

Good luck.

Take care. Nikon does warn every user of the modern DSLR's not to use old flash guns. Old camera's were fitted with mechanical switches. In those days nobody cared about 5, 12 or even 100 volt on a flash gun. Modern camera's use electronic switches (transistor) and they only can survive when hooked up to modern flash guns, because these guns block high (above 5 Volt) voltages.

You may download a copy of the manual from here.

By default the AE/AF Lock only holds while you hold down the AE/AF Lock button on the back of the camera. If you've changed the configuration so that this button locks the AE/AF, you can unlock it either by pressing the button again or by turning the camera off and on again.

I have this same problem. Just started this month. Camera is nearly 1 year old and has had very light use. Sorry, not a fix (yet).

Without specific reference to the D90, the following applies to almost all dSLR's: Skylight filters (UV) are fine, but avoid using auto white balance. Polarisers can prevent the autofocus system from working, but the more expensive circular polarisers are usually OK although don't produce quite the same effect as linear polarisers.

You have auto-bracketing on. It's the little button labled "bkt" by the D90 logo in the front.

http://www.nikon.pl/pl_PL/

The 550 FD came out before digitals. It is fully dedicated(FD) to a type of film camera. Usually this is noted on the back of the flash. CA or C for Canon, MI or MX for Minolta, Ne or NI for Nikon. If its the Nikon dedicated unit, it should not harm your camera, and will most likely work...as a manual flash. You will lose all the neat flash stuff that the D5000 can do.
I suggest you watch Ebay for a Sunpak PZ-30, PZ-40, or a Sigma 430 or 500. The Sigma EF 500 Super will do pretty much everything the top of the line Nikon Flash does, but you can buy it new for half the price of the Nikon unit. All flashes today are dedicated to a certain group of cameras by a certain manufacturer. For example: Canon EOS film cameras or Minolta manual focus cameras.
I suggest you get rid of the 550FD as it is designed for the pre-Autofocus cameras.
Avoid the Sunpak Auto 30DX, the 333, 383, 433, 444 and the PZ 4000/5000 as well as the MZ flashes. These were not designed for your new digital.
Metz also makes some very nice flashes, but they're kinda pricey.

Not compatible.
Get slave triggers and use them as remote flashes, or buy another film camera.
It seems Vivitar no longer is in the flash business. Get a Sigma or Sunpak digital flash for your SX10