It's probably not the oil. 10w-30 and 5w-30 are the same 'weight' oil at operating temp....the only difference between them is when the engine is cold(5-30 is less viscous at colder temperatures and at start up). If oil were the culprit you would really only notice the difference when the engine was 'cold'. It sounds to me like someone may have gotten a bit overzealous and damaged the oil sending unit as Master01 alluded to, or the sender may be just be going bad - parts do wear out. A bad sending unit will usually cause a 'spiked' gauge more than erratic movement. If memory serves, it is quite easy to do on those particular engines. I would have the servicing shop look at it, and get a second opinion from a reputable mechanic. Of course, this is past the fact that there may be something else going on there, so I would have the oil pressure checked by the same mech. as the previous poster suggested, and you may indeed may just simply have a bad dash gauge. Good luck. Let us know what you find. M.