Can I run 1.5V DRAM

it's not said what Optiplex you have.

Memory modules should be installed in pairs of matched memory size, speed, technology, and volts.  If the memory modules are not installed in matched pairs, the computer will continue to operate, but with a slight reduction in performance.

Crucial's compatible RAM for it is 1.35v.  https://www.crucial.com/compatible-upgrade-for/dell/optiplex-3040-small-form-factor 

It all depends on the motherboard.  In general if motherboard spec requires low voltage DDR3L, it would not support regular DDR3.  motherboard simply would not boot because bios detects incompatible DDR3.  if your optiplex spec just says DDR3, then it can run both DDR3 and DDR3L, because DDR3L memory is compatible with DDR3 motherboard.

now since you did not say which you optiplex you have but it uses 240 pin DDR3, I would have to guess the possible models are either (a) or (b).

(a) 780/790/7010/7020, 380/580/980, 390/990, 3010/9010/3020/9020.

all these models use DDR3 and you can also run DDR3L.

or

(b) 3040/5040

these two models require DDR3L.  you cannot run DDR3.

It should be mentioned the list above isn't an entire list of available RAM for the 3040.  A source mentioned too or a link.  While userbenchmark is handy in many ways, not everybody reports their PC build on it.

Thanks so much guys, I didn't know if I should take those specs as the absolute truth, I'm used to HP making silly claims in their documentation.  But I will take your word for it and just get another low voltage stick to match what I already have.  Didn't want to spend the extra money but I can use it with 8GB for a while.  I'll give the Solution to redxps630 even though you both answered the question.    

You are welcome.  Re: what would happen if I ran the 8GB 1.35 Samsung that is in there now and one 8GB 1.5V Micron?

Your pc would not POST if you did that.  I had done that before when trying to install regular ddr3 along a ddr3L already in Dell Inspiron 3650 (which also requires ddr3L).  It did not work.  If you have doubt, you can try it yourself.

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@redxps630 wrote:

You are welcome.  Re: what would happen if I ran the 8GB 1.35 Samsung that is in there now and one 8GB 1.5V Micron?

Your pc would not POST if you did that.  I had done that before when trying to install regular ddr3 along a ddr3L already in Dell Inspiron 3650 (which also requires ddr3L).  It did not work.  If you have doubt, you can try it yourself.

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Thanks again, I wasn't sure about this on a Dell Optiplex, I know most computers will not achieve this but wanted to be sure as my research seemed to say it was possible.  I'm all set now guys, thanks so much.