300SEL is in fact a long wheelbase version of the 300SE, both designated as a 126 chassis. The 300E is the same motor in the 124 chassis. This is the mid-size chassis. All 300 gas motors in this age range are the M103 six. All 126 and 124 chassis cars will have the 3 litre six with some 124 cars available with the 2.6. All the M103's of this era are CIS fuel injected. This is for north america.

The M103 is known for valve seal/guide wear but head gaskets are more of a M104 issue.

Jorg