These last two weeks my car(91 Trans Am GTA) has been sucking a whole lot less fuel than it usually does, I mean it's probably like 25-30% more fuel efficient now and I haven't done anything to it but change the oil recently, surely that can't be it.

About the oil, when I bought the car in germany it had 110,000km on it, I did not find the oil change papers for it so I dunno if it had mineral or synthetic but the mechanic I took it to(for a check up, replace all kinds of fluids and stuff) said I should go with mineral(Quaker State) oil insted of synthetic, I did so.
Then at 119,000km I change the oil and filter at the local mechanic, without me knowing he puts in synthetic(Q8), I changed the oil again recently at 128,000km, again synthetic, this was when I found out I was not running mineral. Still I doubt going from mineral to synthetic is the cause when I have driven for 9000km with it before without knowing.

Usually I think the car gets maybe 16-18 MPG, now it's more like 21-23 mpg.

Could there be something wrong? Injection system, ECM? Or what could be the reason?

I have had problems with a clogged exhaust, old cats broke up and turned into dust, got new ones now but I have still been able to get pieces of old cat out of the exhaust 8 months after the old ones where removed(the new ones are intact, have checked).

I'm not sure but sometimes it almost feels like the car is hickupping, it's hard to tell(maybe clogged exhaust again? but it doesn't feel like it), could as well be bumps in the road, of which there are plenty.

 

maybe if you went from a really thick dino oil right over to synthetic you'd notice that...

My 92 RS had some SUPER thick oil in that thing... I'm guessing 20w-50 or something with some additive.

When I did an oil change with the right wieght I got a couple miles to the gallon more AND I was actually able to notice a slight peppiness in the motor it didn't have before.

All that super heavy oil was slowing down my little ol 3.1.... lol

 

Well the problem there is I changed from dino-oil to synthetic last year and didn't notice a change until maybe a week, half a week after the oil was changed again.

The Q8 I run now is 5W-40, not sure if the earlier synthetic oil was that though.