Us older guys up here in eastern Canada always figured 5w-40 is too thin an oil for these old diesels in any season. Certainly in your areas summer temperatures.

There are a lot of actually minor things to check for hard starting in the winter. I do not want to start an oil thread either.

There is also a chance your new oil de varnished the old lifters a little. Running a 15-40 synthetic oil in the winter and summer is probably okay. 5w-40 may be asking for real trouble.

I would think you'd WANT something like a 5w40 for ease of starting. So what do you use up in eastern Canada?

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14 E250 Bluetec 4Matic "Sinclair", Palladium Silver on Black, 159k miles
06 E320 CDI "Rutherford", Black on Tan, 178k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver on Tan, 145k mi, wastegate conversion, ALDA delete

19 Honda CR-V EX 75k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife