I used to hear myths about being able to go from coventional to sythetic, but not being able to go in reverse order. I dunno, I'm only in my second year of my 4 year degree in chemistry, but it would seem to me if it can mix one way, it can mix the other way too :?
Absolutely. Oil is oil. So called "synthetic oil" came out of the ground just like every other oil, and was refined from the same base stocks. It's just had it's molecular structure altered to make it maintain it's viscosity longer. Regular oil has large and small molecules, and after it has been run through the transmission for a while, the large molecules get chewed up, and the oil gets thinner. Synthetic oil has been run through a centrifuge, which makes all it's molecules about the same size to start with. You can get synthetic oil blends, with regular and synthetic oil mixed together. Synthetic oil is not artificial, it's just been messed with a bit more than regular oil.
But there are reasons to be cautiousin considering it (partcularlly in the direction "conventional" to "synthetic")... As said, they are all mixable and provide lubrication per their ratings, but "non-synthetics" are notas well refined and contain a small amountof light oils and solvents that, over time (years), may expand/swellcertain gasket materials. "Synthetics" containmuch less (or none) of these solvents, so going from "conventional" to "synthetic" may allow, in time, theseals to shrink back, in which case, you may see oil seapage or leaks... (Going from "synthetic" to conventional is less of an issue.. ) Granted,modern seal materials are far less suseptable to this than "old-school" materials.. so little difference is expected.. but it is comething to consider.. and not just theory either.. I have actually experienced this in an old Ford.. tried synthetics, it leaked.. went back to conventional, and the leaks eventually resealed... On something as new as an 1800, I would expect that you would have no problems.
From what I have gathered on the oil syn deal is that in most cases you will get some initial shrinking when changing to syn.
But...as the syn takes effect and has cleaned all the old oil off seals it actually starts to lubricate and make the seals more supple. I did that in a suburban I bought and it developed some small leaks but after the second or third oil change they were gone.