I have a Coleman Powermate 5000/6250 with Tecumseh 10 HP HM-100 engine. Broke in with Castrol HD 30, now keep GC in it for general use, and winter use. But I also keep two quarts of M1 V Twin 20W-50 to be put in it if the ambient will be staying above 80 F. This is basically for the hottest days of summer, like running the whole house AC on a 100 F day.
BTW, I did test that this generator can power the whole house AC. It works ok. The whole house AC unit was the ONLY thing powered up at that time.
Also, on hot days, be sure to use a large box fan to add additional cooling to the electric section of the gen-set. Be sure the direction of flow is not fighting the air flow of the engine.
Post here on BITOG by someone in Florida had info about requirement of generator oil on hot days. 30 wt is not good enough for the really hot days.
I would not run a 40 wt in the winter, not even a multi grade.
For air cooled OPE I stay with SL rated oil. Never SM rated oil.
As for back feeding. If you do try to dump power back into the grid you will be trying to power up the entire neighborhood. If you are lucky the breaker on your genset will pop, but it is just as likely that you will fry your generator. The circuit breaker on these generators is NOT good enough protection to prevent you from damaging it if you over-load it.
It is worth noting that there are expensive inverters used by some people who have solar or wind power that sync with the grid and allow back-feeding and selling power to the grid. This has been legal in the U.S. for many years now.
Yea. if you live out in the country, where there is a long run of line from one house to another there is a small possibility that your house would be disconnected (isolated) during a fault, and if while isolated you fed power to the grid you could kill someone, if they were not following proper procedure.
In general though if you ever connect your generator to the main, you will fry your generator.