I've had to solve lots of oil smells in the cockpit of 757's (RR engines). They mostly occur after TOC indeed. You can easily do some troubleshooting on that by switching off left or right packs. If you have smells with left pack off, it's the right engine that looses some oil.
An engineer can tell this most of the time by removing the spinner cone. Inside there is a tube for hot bleed air (anti ice), and if oil gets in the bleed air, there will be a small puddle of oil inside the spinner. If oil is found engineers will do a ground full power run and check the whole bleed air system. If they also smell it, most probably the engine will be replaced.
You can get oil seepage via the bearings of the engine shafts, they are pressurized by bleed air. Then the smell occurs during engine oil overfill, or with old bearings (as you describe your aircraft as the oldest of the fleet?) We have a company procedure in place that orders us to fill engine oils to 16-17, NEVER to 20-20. That's however a one time Rolls Royce-Boeing-Company agreement because we fly small sectors.
However, if you say it is really a plastic smell, I've only had that once, a mechanic replaced a bleed tube, but did not know some plastic was inside the new one. But then, you would have smells during the whole flight, not only after toc. So, check also if recently a tube was replaced in the trim air system?
Last edited by Piper19; 7th May 2008 at 12:45.