is it possible to tell the difference between regular and synthetic oil? what is the consequences of mixing the two?tia

This might help. http://chemistry.about.com/od/geochemistry/f/What-Happens-If-You-Mix-Regular-And-Synthetic-Oil.htm

I have been using Valvoline VR1 in my engine because it has a higher level of "zinc" which is good for old style engines. So Im told.

See http://www.valvoline.com/faqs/motor-oil/racing-oil for info. Its a conventional oil for racing, but also for street use.

I had synthetic before I changed over. The shop said they did not mix very well. When I changed over I drained it overnight, filled it with 3 ltrs of VR1, ran it for 10 to 15 minutes to mix the oils good and drained it again overnight. Then filled it up with VR1.

i ask because the p o of the tr6 said it had synthetic oil if i remember correctly. also i'm told you don't have to change synthetic oil as often. what's the recommended mile between changes.

I've used RP 20W-40 in both 721 and my newer Z. No issues at all. Change frequency is annually, since the miles driven never approach wearing out the oil. 721 saw a bit of track time, too.