Is there a way to identify what type of oil is in an engine?

After it's been in the system long enough to get dirty, no there's really not. When fresh, synthetic oil "feels" slicker on the hand and is more difficult to wash off with regular hand soap. When burned, it also has a sort of "plastic" smell to it, not sure why, but it has a more acrid smell than conventional.

If in doubt, change it out and put in what you want. If it's been in there long enough so that you don't know what kind of oil it is, it's time for an oil and filter change anyway.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

You might give Blackstone a call. They are an oil testing company. Perhaps they can tell.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Inquiring minds want to know why you ask.

Re: Blackstone, I sent a sample of Rotella Synthetic from my 24V Cummins. They said that I could run the oil a few 1000 more miles. It had about 12,000 on it.

It's nice having to only change once/yr. I didn't send any from the SD because the economics aren't the same but I may send it once to get a baseline reading.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.

I'd bet that any test to determine syn vs. dino would cost as much or more than just changing the oil to what you want.

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617 swapped Toyota Pickup, 22-24 MPG, 50k miles on swap

I had it changed when I got the car 6 mombths ago but forgot to ask or specify. Not sure I trust the shop to remember. I like the way the car is running and there are no leaks. If it's synth I'll stick with it (and I have a quantity of new synthetic for a change). But if it's DuBois that's has been in there, I want to stick with what is working.

No big deal either way, just didn't know if one could determine the two from eachother. Definitely not worth a special test in this case. Thanks!

As long as it's the correct weight and it isn't leaking, what difference does it make? If the engine was designed for conventional oil, either one will work. Wait for the right time to change it and put in whatever you want. If the engine is one of the "Synthetic only" variety, running dino for one oil change isn't going to hurt anything unless you let it go way too long or just beat on the engine.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)