Can I use an oil wash over a clear lacquer topcoat?

...yes...

Almost certainly if  you use mineral spirits to make the wash. I'd avoid turpentine or turpenoid, as these may contain alcohol. If the lacquer is alcohol soluble either of those will attack it.

In the old days it was simple- yes, you can use enamels or enamel thinner (mineral spirits) over lacquer.  But in those old days I talked about, there was only one kind of lacquer, cellulose or related base.

Now, there are acrylic lacquers, and sometimes the bottles or cans merely say lacquer without specifying acrylic lacquer.  I have had problems with enamels and enamel thinners over acrylic lacquer- though some of my friends say I just didn't wait long enough. I don't trust acrylic lacquer anyway, so make sure I get old fashioned real lacquer, or don't try to use enamel products over them.

BTW, the reverse is NOT true.  Do not use lacquers OVER ENAMELS.

Gosh!  Everything is so complex these days!  LOL!  I guess it might help if I were a little bit more specific.  Specifically, I overcoated my P-38 with the Model Master clear gloss lacquer.  Can I use something like the burt umber oil paint as a wash over it or will it attack the clear lacquer?

Thanks!

Eric

I believe that stuff is real lacquer, so you should be okay with paint thinner.  However, since Testors does now sell acrylic lacquer, I would still worry about it. I would paint a piece of plastic, clearcoat it with the lacquer, and then rub it with the paint thinner.  No sense risking a good paint job.