You have to be VERY careful with such "opportunities" to log SIC time
Ultimately YOU (not the company) are responsible for determing what you can log and logging it. Feds, DPE's, and employers are not stupid...they know all the tricks and will catch you if you try the grey area.
Normally, you can only log SIC time in an airplane that is type certified for TWO pilots. Make sure you have the required part 91 training for airplanes that require it. Also, certain airplanes are certified for both single and dual pilot...if the airplane is certified for both, you can always CHOSE to fly two pilot, even if the PIC is single-pilot qualified.
The only exception to this is that you may log SIC in an airplane certified for only one pilot IF (and only if) the airplane is operated under an FAA operating specification which requires an SIC for that airplane and that operation. This is normal in 135 freight operations. But ONLY an FAA OPSPEC can authorize SIC in a single-pilot airplane...insurance requirements and/or "company policy" DO NOT COUNT. In order to do this you will almost certainly need to take a 135 SIC checkride...if you have not done that, it's probably not legal.
You could however ride along on a 135 flight which involves a 91 repositioning leg and fly the repo leg as 91 PIC. Just make sure the other pilot is not ALSO logging PIC in that case...part 91 = one pilot only.