Early condenser design used round copper tubes with 1 or 2 passes and R-12 or R-22 as refrigerant. The size of the r-12 and 22 molecules as well as their ability to collect heat and then release heat allowed AC condenser to be laid down flat function effectively. There were few problems with ‘oil sumping’ e.g. the retention of oil in the condenser and no problems with poor heat transfer away from the system with proper air flow across the condenser. Everything was great.
When R-12 and R-22 was banned and R-134a was introduced. The new R-134 failed to shed heat as easily as R-12 and it did not return the mineral oil back to the compressor as efficiently. The problem lay in the design. Modern condensers use parallel flow condenser. An vertical manifold or a large tube receives the refrigerant from the compressor and distributes across a the core in smaller tubes bisected by heat exchanging fins. The refrigerant and oil pass through the condenser returning to the vehicle to absorb more heat and then to the compressor to lubricate and cool it, and then go around again to continue this process.
This design explanation of multi-flow condensers illustrates why you cannot you lay down multi-flow condenser flat or parallel to the ground. Because oil is heavier and thicker than refrigerant gas the gas separates leaving the oil trapped in the condenser. The refrigerant continues this cycle depositing the oil in the compressor and ultimately starving the compressor of lubrication. Some compressors survive an angled mounting but totally parallel ac condensers coupled with high ambient temperatures and high usage hours will contribute to compressor failure.